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: Ie7daea0d701c7f72c9be95ba530d0f8d68c71cf1
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>