core: drop references to Upstart

The last release of Upstart was July 2014 [1], and there have been no new
commits to the repo. We should move on too.
[1] https://lists.ubuntu.com/archives/upstart-devel/2014-July/003313.html

The only real support was in the code that looked for $PREVLEVEL.
https://codesearch.debian.net/search?q=PREVLEVEL&literal=1&perpkg=1 shows this
string in our own code (or the copy in elogind), our own man pages, and init
scripts for two packages (brltty, salt), which shouldn't be used with systemd.
(And both *check* for PREVLEVEL, and don't set it. So most likely nothing at
all sets it.)

src/systemctl/systemctl.c

@@ -970,8 +970,8 @@ static int parse_argv(int argc, char *argv[]) {
                                 arg_action = _ACTION_INVALID;
                                 return telinit_parse_argv(argc, argv);
                         } else {
-                                /* Hmm, so some other init system is running, we need to forward this request to
-                                 * it. For now we simply guess that it is Upstart. */
+                                /* Hmm, so some other init system is running, we need to forward this request
+                                 * to it. */
 
                                 (void) rlimit_nofile_safe();
                                 execv(TELINIT, argv);