update feature-removal-schedule.txt to include deprecated functions

Now that deprecated functions are detected out of
Documentation/feature-removal-schedule.txt update this to include
kernel_thread.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>