error_notify: Remove custom email error reporting handler.

Restore the default django.utils.log.AdminEmailHandler when
ERROR_REPORTING is enabled.  Those with more sophisticated needs can
turn it off and use Sentry or a Sentry-compatible system.

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit b285813beb)

zerver/lib/logging_util.py

@@ -111,11 +111,6 @@ class ReturnTrue(logging.Filter):
         return True
 
 
-class ReturnEnabled(logging.Filter):
-    def filter(self, record: logging.LogRecord) -> bool:
-        return settings.LOGGING_ENABLED
-
-
 class RequireReallyDeployed(logging.Filter):
     def filter(self, record: logging.LogRecord) -> bool:
         return settings.PRODUCTION