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

This reverts commit 1686594d 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: Ic900d032b622e1b6923b164e1fdd2b7935d0752a
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>