Revert "coredump: hand a pidfd to the usermode coredump helper"

This reverts commit 79bf58c7 which is
commit b5325b2a 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: I6383f380fc1ad663544a3ddb154b193aa0e4eda3
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>