Newbie error adding static caches like mod_forum\search\post::postsdata and friends.

• forum posts are just 1 for each document, no need to cache it, only after MDL-53515 might be worth reusing posts data, but should be unset later after used.
• forum discussions can be very big too
• review other static caches I added in MDL-31989

An alternative for cases where we really need a cache is MUC and request caches, at least admins can choose where to store the data and the main frontend node will not run out of memory