Remove mix test.watch

It recompiled too often and tested too long.

mix.exs

@@ -42,8 +42,7 @@ defp deps do
      {:cachex, "~> 2.1"},
      {:httpoison, "~> 0.11.2"},
      {:ex_machina, "~> 2.0", only: :test},
-     {:credo, "~> 0.7", only: [:dev, :test]},
-     {:mix_test_watch, "~> 0.2", only: :dev}]
+     {:credo, "~> 0.7", only: [:dev, :test]}]
   end
 
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