Changes

testenv-coredump-helper: remove

After looking at this again, it makes more sense to keep
/proc/sys/kernel/core_pattern as "core" on build4 and build5, which we
use for jenkins jobs instead of installing systemd's coredumpctl and
making all coredumps go through that. The motivation for using
coredumpctl in testenv was that lots of developers may have it already
installed (though I'm not sure about that anymore), and that it gives a
nice API for retrieving the related executable name to a coredump
instead of having to parse the output of e.g. "file".

Using coredumpctl had the disadvantage that coredumps for all other
jobs, such as the master-builds are also caught by that and cannot be
easily placed in the workspace directory. I have started implementing
this by extending testenv-coredump-helper to communicate through a
socket and mounting that socket inside docker containers running
contrib/jenkins.sh (host -> lxc -> docker), and it works, but this
complexity is not useful here.

Instead the related patch will make testenv pick the core files when
coredumpctl is not available, and read the path from "file", making
testenv-coredump-helper obsolete.

Related: osmo-ttcn3-hacks Ia765b01432e4cb4cd36c45de874b966e3ebf55bc
Change-Id: I2959c6e6d97d5691ee2a4ae5d49a351eb5811f10