proper fix for fighter/bomber

[Goober5000]

This fixes the fighter/bomber type which had been broken ever since the objecttypes.tbl upgrade in https://github.com/scp-fs2open/fs2open.github.com/commit/246cfcfa655ae58159161011ff1f95dd79d68357: the fighter/bomber type is again treated as a union of fighters and bombers, not a unique type of its own. This requires special handling for the OPF_SHIP_TYPE SEXP argument type, and all four SEXPs that use OPF_SHIP_TYPE have been modified to check for the special case. The explicit fighter/bomber type has again been removed from the built-in objecttypes.tbl.

The requirements for "fighter/bomber" to be valid are 1) there is no explicit "fighter/bomber" type in the mod; 2) a "fighter" type exists; 3) a "bomber" type exists. If any of these conditions is not met, the special case behavior is not enabled.

Follow-up to https://github.com/scp-fs2open/fs2open.github.com/pull/6390. Properly fixes https://github.com/scp-fs2open/fs2open.github.com/issues/6388. All four SEXPs have been tested with both fighter/bomber and non-fighter/bomber types.