Revert "exec: fix up /proc/pid/comm in the execveat(AT_EMPTY_PATH) case"

This reverts commit dc5da273 which is
commit 543841d1 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: Ia20b05965a5beab070ea99a8309f7bf72ae74a4e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>