Remove unused/unsupported code now that the Central Repository feature is released

[marckhouzam]

Bug description

After introducing the Central Repository feature, there is a lot of code paths that are no longer supported.
That code was kept as the Central Repository feature evolved, but now that it has been released, those code paths just complicate the code base for nothing.

Things that can be removed:

• usage of context-target-v2 and no-central-repo-test-only feature flags, and all associated and unused code. The CLI does not support changing the default value of either of these feature flags.
• unused GCP discovery source

Expected behavior

Unused/unsupported code paths are removed to simplify the code base.

[marckhouzam]

A bunch of cleanup was done for v1.1.0 but there is still a bit more than can be done.
I'll keep this open for v1.2.0