email_mirror: Remove unnecessary EMAIL_GATEWAY_PATTERN logging.

These logs were pretty spammy, and there have long been much better
ways to communicate to system administrators that the incoming email
gateway is great, including, most importantly, in the section of the
emails themselves that explains how replying works.

zerver/lib/email_mirror.py

@@ -155,11 +155,10 @@ def get_usable_missed_message_address(address: str) -> MissedMessageEmailAddress
 
 
 def create_missed_message_address(user_profile: UserProfile, message: Message) -> str:
+    # If the email gateway isn't configured, we specify a reply
+    # address, since there's no useful way for the user to reply into
+    # Zulip.
     if settings.EMAIL_GATEWAY_PATTERN == "":
-        logger.warning(
-            "EMAIL_GATEWAY_PATTERN is an empty string, using "
-            "NOREPLY_EMAIL_ADDRESS in the 'from' field."
-        )
         return FromAddress.NOREPLY
 
     mm_address = MissedMessageEmailAddress.objects.create(