Disable "No mixfile found in project..." notification when working on exs files

[Lindenk]

When working on a one off script, the extension publishes a No mixfile found in project. To use a subdirectory, set 'elixirLS.projectDir' in your settings notification each time the file is saved.

Expected Behavior: Do not spam "No mixfile found" messages when working on a script not using mix. A config option to disable the message entirely could also be an option.

Environment

• Elixir & Erlang versions (elixir --version):
  Erlang/OTP 23 [erts-11.1.3] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe]
  Elixir 1.11.0 (compiled with Erlang/OTP 23)

• Operating system: Arch Linux

• Editor or IDE name (e.g. Emacs/VSCode): VSCode

Troubleshooting

• Restart your editor (which will restart ElixirLS) sometimes fixes issues
• Stop your editor, remove the entire .elixir_ls directory, then restart your editor

[lukaszsamson]

Duplicate of #307