compositor: Stop trapping SIGQUIT

We've been trapping SIGQUIT for a "clean shutdown" since commit 3cad436a

However, sources such as:
http://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html


indicate we probably shouldn't be trapping it at all, as the intent of
SIGQUIT is to leave a core file and debug artifacts from the run.

We should perform the minimal amount of clean up to ensure the system isn't
left in an unusable state - but these days that's performed by other
software such as logind.

We can safely stop trapping SIGQUIT entirely.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>