sysctl: make CONFIG_SYSCTL_SYSCALL default to n

sysctl: make CONFIG_SYSCTL_SYSCALL default to n

When I tried to send a patch to remove it, Andi told me we still need to
keep compabitlies for old libc, so we can't remove this completely.  Then
just make it default to n and remove the doc from
feature-removal-schedule.txt.

Signed-off-by: WANG Cong <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>