Remove an unnecessary global

The _DISCUSSIONINFO global was originally used as a cache, but has since
lost that capability and is therefore just harmful. This is a precursor
to more refactoring that will improve the performance of the forums and
may itself provide some performance improvement because it separates the
computation done by two functions that each previously computed the
entirety of _DISCUSSIONINFO.
Improve performance of forum views

Avoid recomputing course module information for every thread, which
should dramatically improve the performance of high-percentile latency
queries.

JIRA: FOR-250