Fix mirror mode breaking merge bones

[989onan]

Mirror mode being on breaks merge bones by auto deleting or mirroring the armature. We can let the end user merge the other side manually, instead of having them depending on mirror and merging one side and expecting cats to merge the other and/or account for mirror mode.

[Mysteryem]

Please restore the mirror settings back to their original values once done.

And putting this change into set_default_stage does not seem like a good idea, why would it be needed there? If a specific operator needs specific state, temporarily set that state only for that operator. We shouldn't be needlessly messing with settings set by users like this.

[989onan]

closing due to abandoned repo