remove documentation about broken/depricated mopidy-spotify plugin & remove trailing whitespace

[MrMineDe]

Removing the documentation about the broken mopidy-spotify plugin. The plugin needed an depricated api from spotify and the chance that mopidy-spotify will work in the near future is quite small, therefor the documentation is misleading.

Potentially this can be followed up by replacing the documentation that is removed with this pull request by something more general about mopidy plugins or about working alternatives like mopidy-ytmusic

Also this removes all the trailing whitespace at the end of lines in README