Fix formation merging issues.
The motion parameters were calculated even without members, which caused us to try set a undefined passclass.
Also were members added to previously merged twin-formations. Once merged (i.e. disbanded), they are moved out of world now.
Some cleanups/deduplication whilst at it.
Reported by: @andy5995 at the forums (https://wildfiregames.com/forum/topic/71578-feedbacks-from-a26-svn-tests/page/8/#comment-505078)
Differential revision: https://code.wildfiregames.com/D4727
Comments by: @Langbart, @marder
Tested by: @Langbart