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

This reverts commit cdb61a70 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: I9ab9c7ee427a717644d3bce8f6a6b89839658be1
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>