mpd: Remove "Comment" tag type from translator output.

Newer versions of the protocol have removed this tag, so we should as well.
This also works around the issue of #881 which was breaking things with
newlines in comment fields.

The readcomments command seems to replace this, but it seems to only care about
specific extra tagtypes, not the general comment tag we normally collect when
scanning things.
mpd: Remove newline escaping code.

This was added for #881, where the correct fix turned out to be to remove
comments from the responses. We should still add some sanity checks for
verifying that our responses at the very least only contain printable chars.
Merge pull request #907 from adamcik/feature/mpd-remove-comments

Feature/mpd remove comments