sysctl: remove obsolete comments

ctl_unnumbered.txt have been removed in Documentation directory so just
also remove this invalid comments

[[e-mail]: fix Documentation/sysctl/00-INDEX, per Dave]
Signed-off-by: Jovi Zhang <[e-mail]>
Cc: Dave Young <[e-mail]>
Acked-by: WANG Cong <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/sysctl/00-INDEX

···
 	- general information about /proc/sys/ sysctl files.
 abi.txt
 	- documentation for /proc/sys/abi/*.
-ctl_unnumbered.txt
-	- explanation of why one should not add new binary sysctl numbers.
 fs.txt
 	- documentation for /proc/sys/fs/*.
 kernel.txt

kernel/sysctl.c

···
 		.mode		= 0555,
 		.child		= dev_table,
 	},
-/*
- * NOTE: do not add new entries to this table unless you have read
- * Documentation/sysctl/ctl_unnumbered.txt
- */
 	{ }
 };
 
···
 		.proc_handler	= proc_dointvec,
 	},
 #endif
-/*
- * NOTE: do not add new entries to this table unless you have read
- * Documentation/sysctl/ctl_unnumbered.txt
- */
 	{ }
 };
 
···
 		.extra2		= &one,
 	},
 #endif
-
-/*
- * NOTE: do not add new entries to this table unless you have read
- * Documentation/sysctl/ctl_unnumbered.txt
- */
 	{ }
 };
 
···
 		.proc_handler	= &pipe_proc_fn,
 		.extra1		= &pipe_min_size,
 	},
-/*
- * NOTE: do not add new entries to this table unless you have read
- * Documentation/sysctl/ctl_unnumbered.txt
- */
 	{ }
 };