Stop generating Sigstore disjoint verification materials (crt/sig)

[sethmlarson]

sigstore-python v1 currently generates disjoint verification materials (.crt/.sig) and bundles (.sigstore). v2 only generates bundles by default, and bundles are going to be the verification material of choice going forward.

Once python/pythondotorg#2300 has landed and been documented we can remove the generation of disjoint verification materials from release-tools, instead only generating bundles.