Remove gen-sdk vs legacy comparison tooling after full migration

[pose]

Context

Part of the gen-sdk migration epic, pulumi/home#4787. The shadow-gen diff tooling (#2282) and the generated make compare_sdk_<lang> target exist only to prove, per provider+language, that pulumi package gen-sdk output matches the legacy per-provider codegen during the migration. They are transitional scaffolding, not a permanent part of the build.

Scope

Once every provider+language has been flipped to pulumi package gen-sdk and verified, remove the comparison mechanism entirely:

• the compare_sdks / compare_sdk_<lang> targets from the base Makefile template (the section is marked temporary in-place),
• the provider-ci compare-sdk subcommand and the internal/pkg/comparesdk package,
• the comparison CI job/workflow,
• the aggregate parity reporting (Aggregate gen-sdk parity across providers from per-repo compare results #2287).

If the legacy codegen path is fully retired at the same time, the now-unused useXPackageGenSdk opt-in flags can be removed too (or tracked as a follow-up).

Acceptance criteria

• Comparison tooling, make targets, CI job, and aggregate reporting removed from ci-mgmt.
• Regenerated provider repos no longer carry the temporary Makefile section.
• Gated on all providers being verified as migrated (tracked via the epic's per-provider status).

Part of pulumi/home#4787.