kbuild: delete INSTALL_FW_PATH from kbuild documentation

Removed Kbuild documentation for INSTALL_FW_PATH.

The kbuild symbol INSTALL_FW_PATH was removed from Kbuild tools in
September 2017 (for 4.14) but the symbol was not deleted from
the kbuild documentation, so do that now.

Fixes: 5620a0d1aacd ("firmware: delete in-kernel firmware")
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: [e-mail] # 4.14+
Cc: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/kbuild/kbuild.txt

···
 the default option --strip-debug will be used.  Otherwise,
 INSTALL_MOD_STRIP value will be used as the options to the strip command.
 
-INSTALL_FW_PATH
---------------------------------------------------
-INSTALL_FW_PATH specifies where to install the firmware blobs.
-The default value is:
-
-    $(INSTALL_MOD_PATH)/lib/firmware
-
-The value can be overridden in which case the default value is ignored.
-
 INSTALL_HDR_PATH
 --------------------------------------------------
 INSTALL_HDR_PATH specifies where to install user space headers when