[ticket/12505] Remove outdated media handing in attachments

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

PHPBB3-12505
[ticket/12505] Remove outdated code from prosilver

Remove outdated code from prosilver style

PHPBB3-12505
[ticket/12505] Remove outdated code from subsilver2

Remove outdated code from subsilver2 style

PHPBB3-12505
[ticket/12505] Remove outdated media handling from acp

Remove outdated media handling from acp

PHPBB3-12505
[ticket/12505] Move outdated media to downloadable files

Update constants and data schema

PHPBB3-12505