Partially revert "home: Enable partial_subscribers in web app."

This reverts commit 458c660f65.

We realized we need one more pull request before we can turn this on.
I kept the hunks related to finalizing the cutoffs.

zerver/lib/home.py

@@ -1,4 +1,5 @@
 import calendar
+import os
 import time
 from dataclasses import dataclass
 from urllib.parse import urlsplit
@@ -109,7 +110,7 @@ def build_page_params_for_home_page_load(
     if user_profile is not None:
         client = RequestNotes.get_notes(request).client
         assert client is not None
-        partial_subscribers = True
+        partial_subscribers = os.environ.get("PARTIAL_SUBSCRIBERS") is not None
         state_data = do_events_register(
             user_profile,
             realm,