mpd & mpdscribble: fix plist

I should not have put those keys in there to begin with.

These lead to the plists not working on Yosemite (because the `#{var}/log` dir is owned by root, with the very obscure error `com.apple.xpc.launchd[1]: (homebrew.mxcl.mpd[931]) Service could not initialize: 14A329f: xpcproxy + 14049 [1344][4D240BE7-AF8F-3396-859F-B6E646073F3A]: 0xd
`) and don't provide much additional value since both mpd and mpdscribble allow specifying a log file in their respective config files.

Sorry for the hassle :sweat_smile: