Remove the custom SIGINT and SIGTERM handlers

[isum]

This PR removes the custom SIGINT and SIGTERM signal handles, allowing the default OS behavior.

Custom signal handlers prevent graph-node from shutting down on SIGINT and SIGTERM signals because not all of its components handle the signals.

[lutter]

I don't know if it's worth keeping this code around - we can always resurrect it from git history if we do need it.