Remove http/static_dir support

[jodal]

The http/static_dir config value was deprecated in Mopidy 1.0 almost a year ago. We should remove it entirely because:

• Properly packaged web clients are much easier to install for end users.
• http/static_dir can only be used by a single client.
• http/static_dir hides the list of other installed clients (that is, it is always available at http://.../mopidy/).
• Our cookiecutter project makes is easy getting a properly packaged web client up and running, eliminating then need for http/static_dir as a development tool.

Tasks:

• Change the config schema type to config_lib.Deprecated()
• Remove default from mopidy/http/ext.conf
• Remove static dir hosting support in Mopidy-HTTP (see _get_mopidy_request_handlers())
• Update JS extension docs to not use http/static_dir in examples