streams: Remove is_stream_admin property and its uses.

This commit removes the is_stream_admin property of Subscription
model and also updates check_stream_access_for_delete_or_update
to not return true when is_stream_admin is True.

We also removes the relevant tests.

This change is done as we would not be moving forward with the
stream administrator concept as we have decided to modify the
permissions model as per #19525.

zerver/lib/streams.py

@@ -312,9 +312,6 @@ def check_stream_access_for_delete_or_update(
     if sub is None and stream.invite_only:
         raise JsonableError(error)
 
-    if sub is not None and sub.is_stream_admin:
-        return
-
     raise OrganizationAdministratorRequired()