Remove obsolete code from BBCodes ACP

With the BBCode routines replaced in 3.2, a large chunk of code related to regexps can be removed from the ACP module in 3.3.

Replacing the regexps previously used to render custom BBCodes with non-matching, valid regexps has two immediate effects:

1. It prevents PHP warnings related to the e flag.
2. It prevents custom BBCodes from being rendered at all in old posts until they are updated by the 3.2 reparser. Custom BBCodes will appear as plain text. Default BBCodes will work normally.