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

This reverts commit 510cf09f 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: I4a79afe6af53e2a6384faa9edad4736391d1b2a1
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>