Fix dialyzer error in Mix.Tasks.Absinthe.Schema.Json

Remove elze part of with expression in generate_schema. Dialyzer
saw that Absinthe.Schema.introspect will only return
{:error, String.t()} if there is an error

Author: bryanjos

lib/mix/tasks/absinthe.schema.json.ex

@@ -101,9 +101,6 @@ defmodule Mix.Tasks.Absinthe.Schema.Json do
     with {:ok, result} <- Absinthe.Schema.introspect(schema) do
       content = json_codec.encode!(result, pretty: pretty)
       {:ok, content}
-    else
-      {:error, reason} -> {:error, reason}
-      error -> {:error, error}
     end
   end