Get rid of GROUP_*_NO_CREDIT and use no_credit

The no_credit field of struct GROUP is used instead the conditional
compilation based on GROUP_*_NO_CREDIT symbols. These symbols are
removed from lintypes.h, and they are no longer used in engine.c and
mouse.c.

Fix: The university no_credit field has to be set to 'true'.

The no_credit_build_msg() function in mouse.c is refactored (although it
must be refactored again to be generic and don't hardwire the groups
involved).

The new no_credit_build() function in engine.c it's only 2 comparations
and only used in one place, so it's replaced by its body and deleted.