weston: remove SEGV and ABRT handlers

Catching an ABRT is kind of ok, catching a SEGV is russian roulette. We
have been quite lucky with it, but I've started hitting crashes inside
malloc() which causes a deadlock when our SEGV handler needs to malloc()
as well (weston_log_timestamp()).

One reason to catch SEGV and ABRT was to attempt to restore the VT on
the DRM-backend. Nowadays that job is done by logind or weston-launch.

The signal handler also printed a backtrace, which for me personally has
been extremely helpful. Arguably it's not necessary though, when we have
core files and services that catch cores. For instance, if using
systemd, 'coredumpctl gdb' is delightfully easy for getting into the
saved core.

Therefore, this code does more harm than it is useful, so remove it. We
also drop an optional dependency to libunwind.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>