fixes a performance problem in one sql select

the "select *" in the original version has a huge performance penalty, since the course table has lots of columns with lots of data (esp. in columns like modinfo, containing several KB to MB per row in our installation). transferring all that data takes a lot of network time. only requesting the columns that are actually needed in this function sped up the request by a factor of around 100 (i.e. 1 second to 1/100 second).