realm: Create RealmAuditLog entry when removing realm linkifiers.

This commit also adds 'acting_user' parameter to do_remove_linkifier
function.

Fixes a part of #21268.

zerver/models.py

@@ -4255,6 +4255,7 @@ class AbstractRealmAuditLog(models.Model):
     REALM_PLAYGROUND_REMOVED = 222
     REALM_LINKIFIER_ADDED = 223
     REALM_LINKIFIER_CHANGED = 224
+    REALM_LINKIFIER_REMOVED = 225
 
     SUBSCRIPTION_CREATED = 301
     SUBSCRIPTION_ACTIVATED = 302