Remove obsolete check in SIGTERM handler for the startup process.

Remove obsolete check in SIGTERM handler for the startup process.

Thanks to commit 3b00fdba9f, this check in the SIGTERM handler for
the startup process is now obsolete and can be removed.  Instead of
leaving around the dead function write_stderr_signal_safe(), I've
opted to just remove it for now.

This partially reverts commit 97550c0711.

Reviewed-by: Andres Freund, Noah Misch
Discussion: https://postgr.es/m/20231121212008.GA3742740%40nathanxps13