DB::Discussions: Minor improvement to post <-> board filter query
DB::Discussions: Minor improvement to post <-> board filter query

I don't know why I didn't apply this one before, I did make this change
when benchmarking the fulltext search queries and with the introduction
of the bb_tsvector() function this change pretty much always improves
performance.

christian-roggia/vndb,erengy/vndb,morkt/vndb,morkt/vndb,morkt/vndb,christian-roggia/vndb,erengy/vndb,morkt/vndb,morkt/vndb,erengy/vndb,christian-roggia/vndb