Revert "Bluetooth: MGMT: Protect mgmt_pending list with its own lock"

This reverts commit 4e83f2db which is
commit 6fe26f69 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: Ia71be8ae26f83011527d07fbbfdf9d6d282f23a4
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>