aircombat.c isn't used for intercepting missiles, simplify
Chainsaw had missiles flying missions together with planes, and to
make that work, aircombat.c got code for coping with missiles. Since
Empire 2, missiles fly separately and don't go through aircombat.c
anymore. The missile code there has been useless for more than a
decade. Remove it.
This simplifies ac_encounter(), sam_intercept(), ac_intercept(),
ac_airtoair(), and gets rid of count_non_missiles(), all_missiles().