core: remove unused exception

lib/rdf/exceptions.ex

@@ -46,15 +46,3 @@ end
 defmodule RDF.Namespace.UndefinedTermError do
   defexception [:message]
 end
-
-
-defmodule RDF.InvalidRepoURLError do
-  defexception [:message, :url]
-
-  def exception(opts) do
-    url = Keyword.fetch!(opts, :url)
-    msg = Keyword.fetch!(opts, :message)
-    msg = "invalid url #{url}, #{msg}"
-    %__MODULE__{message: msg, url: url}
-  end
-end