Since commit bcf9033e ("sched: move CPU field back into thread_info
if THREAD_INFO_IN_TASK=y"), the CPU field in thread_info went back to
being managed by the core code, so we no longer have to keep it in sync
in arch code.

While at it, mark THREAD_INFO_IN_TASK as done for ARM in the
documentation.
Reviewed-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NArd Biesheuvel <ardb@kernel.org>
Signed-off-by: NRussell King (Oracle) <rmk+kernel@armlinux.org.uk>