CLEANUP: server: make it clear that srv_check_for_deletion() is thread-safe

CLEANUP: server: make it clear that srv_check_for_deletion() is thread-safe

This function was marked as requiring thread isolation because its code
was extracted from cli_parse_delete_server() and was running under
isolation. But upon closer inspection, and using atomic loads to check
a few counters, it is actually safe to run without isolation, so let's
reflect that in its description.

However, it remains true that cli_parse_delete_server() continues to call
it under isolation.