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

This reverts commit 4ab90190 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: I91f8ae942e97b716f29bc0344e90863edf427e63
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>