Documentation: Mark the 'efivars' sysfs interface as removed

The 'efivars' sysfs interface was removed in commit 0f5b2c69a4cb ("efi:
vars: Remove deprecated 'efivars' sysfs interface"), but the ABI
documentation was not updated properly.

Strip down the documentation file for /sys/firmware/efi/vars to a very
basic description of what the interface was about, add a section about
the rough removal timeline, and inform the reader about the intended
replacement.

Signed-off-by: Tim Schumacher <[e-mail]>
Signed-off-by: Ard Biesheuvel <[e-mail]>

Documentation/ABI/removed/sysfs-firmware-efi-vars

···
+What:		/sys/firmware/efi/vars
+Date:		April 2004, removed March 2023
+Description:
+		This directory exposed interfaces for interacting with
+		EFI variables.  For more information on EFI variables,
+		see 'Variable Services' in the UEFI specification
+		(section 7.2 in specification version 2.3 Errata D).
+
+		The 'efivars' sysfs interface was removed in March of 2023,
+		after being considered deprecated no later than September
+		of 2020. Its functionality has been replaced by the
+		'efivarfs' filesystem.

Documentation/ABI/stable/sysfs-firmware-efi-vars

···
-What:		/sys/firmware/efi/vars
-Date:		April 2004
-Contact:	Matt Domsch <[e-mail]>
-Description:
-		This directory exposes interfaces for interactive with
-		EFI variables.  For more information on EFI variables,
-		see 'Variable Services' in the UEFI specification
-		(section 7.2 in specification version 2.3 Errata D).
-
-		In summary, EFI variables are named, and are classified
-		into separate namespaces through the use of a vendor
-		GUID.  They also have an arbitrary binary value
-		associated with them.
-
-		The efivars module enumerates these variables and
-		creates a separate directory for each one found.  Each
-		directory has a name of the form "<key>-<vendor guid>"
-		and contains the following files:
-
-		=============== ========================================
-		attributes:	A read-only text file enumerating the
-				EFI variable flags.  Potential values
-				include:
-
-				EFI_VARIABLE_NON_VOLATILE
-				EFI_VARIABLE_BOOTSERVICE_ACCESS
-				EFI_VARIABLE_RUNTIME_ACCESS
-				EFI_VARIABLE_HARDWARE_ERROR_RECORD
-				EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS
-
-				See the EFI documentation for an
-				explanation of each of these variables.
-
-		data:		A read-only binary file that can be read
-				to attain the value of the EFI variable
-
-		guid:		The vendor GUID of the variable.  This
-				should always match the GUID in the
-				variable's name.
-
-		raw_var:	A binary file that can be read to obtain
-				a structure that contains everything
-				there is to know about the variable.
-				For structure definition see "struct
-				efi_variable" in the kernel sources.
-
-				This file can also be written to in
-				order to update the value of a variable.
-				For this to work however, all fields of
-				the "struct efi_variable" passed must
-				match byte for byte with the structure
-				read out of the file, save for the value
-				portion.
-
-				**Note** the efi_variable structure
-				read/written with this file contains a
-				'long' type that may change widths
-				depending on your underlying
-				architecture.
-
-		size:		As ASCII representation of the size of
-				the variable's value.
-		=============== ========================================
-
-
-		In addition, two other magic binary files are provided
-		in the top-level directory and are used for adding and
-		removing variables:
-
-		=============== ========================================
-		new_var:	Takes a "struct efi_variable" and
-				instructs the EFI firmware to create a
-				new variable.
-
-		del_var:	Takes a "struct efi_variable" and
-				instructs the EFI firmware to remove any
-				variable that has a matching vendor GUID
-				and variable key name.
-		=============== ========================================

Documentation/filesystems/efivarfs.rst

···
 *See also:*
 
 - Documentation/admin-guide/acpi/ssdt-overlays.rst
-- Documentation/ABI/stable/sysfs-firmware-efi-vars
+- Documentation/ABI/removed/sysfs-firmware-efi-vars