models: Delete unused UserProfile.is_service_bot.

This function had the wrong format for its use case (in
zerver/lib/actions.py) anyway, since we don't have a full UserProfile
object there.

zerver/models.py

@@ -772,10 +772,6 @@ class UserProfile(AbstractBaseUser, PermissionsMixin):
     def is_embedded_bot(self) -> bool:
         return self.bot_type == UserProfile.EMBEDDED_BOT
 
-    @property
-    def is_service_bot(self) -> bool:
-        return self.is_bot and self.bot_type in UserProfile.SERVICE_BOT_TYPES
-
     @property
     def allowed_bot_types(self) -> List[int]:
         allowed_bot_types = []