audio/mixxx: Unbreak by removing %%DATADIR%% files from pkg-plist, because PORTDATA=* exists

Other port changes:

• Remove DEPRECATED: it doesn't use QT4 at all (it was there by mistake)
• Add USES=gl
• Delete empty directory that was breaking polist when all options are off
• Change WWW to https

Reported by: poudereie failure
Approved by: portmgr (unbreak)