Remove outdated media handing in attachments

Remove code for handling media in RealPlayer, QuickTime, and Windows Media
Player.

PHPBB3-12505
Remove outdated code from prosilver

PHPBB3-12505
Remove outdated code from subsilver2

PHPBB3-12505
Remove outdated media handling from acp

PHPBB3-12505
Move outdated media to downloadable files

Update constants and data schema

PHPBB3-12505