ARM: 9158/1: leave it to core code to manage thread_info::cpu

Since commit bcf9033e54 ("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: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Documentation/features/core/thread-info-in-task/arch-support.txt

@@ -8,7 +8,7 @@
     -----------------------
     |       alpha: | TODO |
     |         arc: | TODO |
-    |         arm: | TODO |
+    |         arm: |  ok  |
     |       arm64: |  ok  |
     |        csky: | TODO |
     |       h8300: | TODO |