Fix #2394 - Crash when remove armature object + export only def bones

[julienduroure]

Fix #2394

This check is done before the filter of non deformation bone.
So when we want to get this list, as it's cached, we get the list with non deformation bones, so try to access a bone that is filtered

Additional bug when we want to export only DEF bones :
the check is done on all bugs, not only deformation bones, if we want to get only def bones.
So having at root a DEF and a nonDEF bone => Check will failed and said we can't remove armature

Solution: When checking if we can remove armature, indicate that we don't want to cache the result