Don't remove parts that aren't there.

this does not fix the underlying problem,
which is that the way vehicle parts are handled is insane.

specifically remove_part() removes by the index of the part in the parts list, but removing a part modifies the parts list, which changes the meaning of every index not less than the removed part's index in the old part list, thus causing insanity.

see also https://github.com/CleverRaven/Cataclysm-DDA/pull/4169#issuecomment-29569838