Revert "drop_monitor: Require 'CAP_SYS_ADMIN' when joining "events" group"

This reverts commit 4a341627 which is
commit e03781879a0d524ce3126678d50a80484a513c4b 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: Iecbd6b6537bd4cd2d178d0afbdc7557e521429c5
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>