Pass only PM_* mission flags to ac_encounter()

Pass only PM_* mission flags to ac_encounter()

Since the previous two commits, ac_encounter() checks its
mission_flags argument only for proper mission flags PM_R and PM_S,
not for plane flags P_A, P_S, P_I.

This makes the code to put plane flags into mission flags useless.
Remove it from bomb(), drop(), fly(), para(), reco(),
perform_mission(), mission_pln_arm(), air_defense(), pln_arm().

Much of that code was useless even before: P_X and P_H since Chainsaw
3 option STEALTHV became mandatory in Empire 2, and P_MINE since
commit cc0c3e4f (v4.3.0) cleaned up mine drops.