remove discord link

Author: mceachen

SECURITY.md

@@ -10,8 +10,7 @@ If you find a vulnerability, _or even think you have_, please send an email to
 the author. Each signed git release by `mceachen` contains a monitored email
 address.
 
-If you prefer, you may DM [@PhotoStructure on Discord](https://discord.com/users/photostructure) or
-[@mrm on the PhotoStructure forum](https://forum.photostructure.com/new-message?username=mrm&title=Security+concern).
+If you prefer, you may [DM @mrm on the PhotoStructure forum](https://forum.photostructure.com/new-message?username=mrm&title=Security+concern).
 
 Expect a reply in under 36 hours. Please try an alternative contact method if
 you don't get a response.