Removing the xmp.dc.subject (and itpc.keywords?) tags won’t help with the tags of form <t>|<t>|..., those have to come from xmp.lr.hierarchical or another field storing a hierarchy.
But in Digikam you can select which fields to write to sidecars, in settings->metadata->advanced.
That should allow you to at least get rid of the indivual tags. I don’t think you can get rid of the composite tags that are stored outside the tree that way, though.