cleanup check_victory in ai actions

do_execute which calls run_in_synced_context can throw end_level
exception because run_in_synced context calls check_victory. So we don't
need the additional check_victory in action_result::execute anymore.
remove check_victory in play_human_turn

we don't need these because we call check_victory at the end of the
synced_context.

I removed them because check_victory assumes that "this code is run
rarely" which is not the case if we call it there.