Revert "Add support to deal with album arts sent by mpris plugin."

This was very poorly implemented and can't stay as it is right now:

• Every second or so the art image was being loaded from disk, scaled, base64 encoded and sent over the freakin network!
• The Android interface didn't take into account small screens, and adding the image would cut stuff out of the screen.
• Didn't manage "edge cases" like playing a song without cover after one with cover (previous image was still being shown) or changing players.

This reverts commit 24c404400f6e5092d38d4e26a948777c1545ca2b.

1. Conflicts:
2. src/org/kde/kdeconnect/Plugins/MprisPlugin/MprisActivity.java
3. src/org/kde/kdeconnect/Plugins/MprisPlugin/MprisPlugin.java