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

This reverts commit ac190912 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: I79cda13392f475443cb9b0aa701b9224df1fa39e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>