refactor battle_context

this has multiple advantages:;
1) it fixes #3594, an assertion failue that happend when a unit with
only one attack attacked when that one attack was disabled.
2) It now creates each pair or battle_context_unit_stats only once for
each pauir of attacks, previsouly the coud would create multiple
temporary battle_context_unit_stats objects to for exampel check
"disabled"
3) It simplifies the code by removing special handling of some cases.