Retire MASTER_SITE_MPLAYERHQ

Rationale:

• MPLAYERHQ was an alias for www[1-5].mplayerhq.hu
• www.mplayerhq.hu uses dns load balancing already for a while, essentially rendering the above alias futile
• At times, some of the www[1-9] instances do not participate in the balancing while others do. (At the time of this writing, for instance www3 does not have a dns record but www8 does. The latter was not part of MASTER_SITE_MPLAYERHQ)
• The MPLAYERHQ alias was only used by two ports in the tree: multimedia/mplayer-skins multimedia/win32-codecs