Bug 1547756 - remove support for -register commandline flag because it's bogus in 2019, r=froydnj

Apparently introduced by bryner back in 2002, and refactored numerous times without too much scrutiny, this:
- presumably was there for binary components (which are no longer supported)
- or external (add-on) restart-required components (ditto)
- or really any non-builtin components (ditto)
- and would presumably at one point store data somewhere... except that's kinda hard without a profile.
- seems to have never been documented, as far as I can tell.

So let's remove it.

Differential Revision: https://phabricator.services.mozilla.com/D29229

--HG--
extra : moz-landing-system : lando

toolkit/xre/nsAppRunner.cpp

@@ -3399,30 +3399,6 @@ int XREMain::XRE_mainInit(bool* aExitFlag) {
   rv = XRE_InitCommandLine(gArgc, gArgv);
   NS_ENSURE_SUCCESS(rv, 1);
 
-  // Check for --register, which registers chrome and then exits immediately.
-  ar = CheckArg("register", nullptr,
-                CheckArgFlag::CheckOSInt | CheckArgFlag::RemoveArg);
-  if (ar == ARG_BAD) {
-    PR_fprintf(PR_STDERR,
-               "Error: argument --register is invalid when argument --osint is "
-               "specified\n");
-    return 1;
-  }
-  if (ar == ARG_FOUND) {
-    ScopedXPCOMStartup xpcom;
-    rv = xpcom.Initialize();
-    NS_ENSURE_SUCCESS(rv, 1);
-    {
-      nsCOMPtr<nsIChromeRegistry> chromeReg =
-          mozilla::services::GetChromeRegistryService();
-      NS_ENSURE_TRUE(chromeReg, 1);
-
-      chromeReg->CheckForNewChrome();
-    }
-    *aExitFlag = true;
-    return 0;
-  }
-
   return 0;
 }