Revert "sched: Add wrapper for get_wchan() to keep task blocked"

This reverts commit 5ce1264b which is
commit 42a20f86 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ibb1a89fea36630457207804654ac4ad2dd51047e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>