bots: Eliminate NEW_USER_BOT.

This bot was basically a duplicate of NOTIFICATION_BOT for some
specific corner cases, and didn't add much value.  It's better to just
eliminate it, which also removes some ugly corner cases around what
happens if the user account doesn't exist.

zerver/views/home.py

@@ -126,7 +126,9 @@ def home_real(request: HttpRequest) -> HttpResponse:
 
     # Reset our don't-spam-users-with-email counter since the
     # user has since logged in
-    if user_profile.last_reminder is not None:
+    if user_profile.last_reminder is not None:  # nocoverage
+        # TODO: Look into the history of last_reminder; we may have
+        # eliminated that as a useful concept for non-bot users.
         user_profile.last_reminder = None
         user_profile.save(update_fields=["last_reminder"])
 
@@ -172,7 +174,6 @@ def home_real(request: HttpRequest) -> HttpResponse:
     # These end up in a global JavaScript Object named 'page_params'.
     page_params = dict(
         # Server settings.
-        new_user_bot_configured = settings.NEW_USER_BOT is not None,
         development_environment = settings.DEVELOPMENT,
         debug_mode            = settings.DEBUG,
         test_suite            = settings.TEST_SUITE,