[MM-21020] Return same error when creating a duplicate channel (#13429)

* Return same error when trying to create a channel with the same name of an existing one

* Remove unused arguments

i18n/en.json

@@ -5942,10 +5942,6 @@
     "id": "store.sql_channel.save_channel.limit.app_error",
     "translation": "You've reached the limit of the number of allowed channels."
   },
-  {
-    "id": "store.sql_channel.save_channel.previously.app_error",
-    "translation": "A channel with that URL was previously created"
-  },
   {
     "id": "store.sql_channel.save_channel.save.app_error",
     "translation": "Unable to save the channel"