Refactor ApiRegistrar class away

[gerlowskija]

Description

Earlier on during v2 API refactoring, I had created ApiRegistrar as a way to bundle together the registration of API classes that were multiplying and cluttering up CoreContainer.load.

In hindsight, this was based on a misunderstanding that there was a reason to register APIs (directly) in CoreContainer.load in the first place. But there's not: both core and container-level APIs can be registered just fine via the 'getApis()' method present on their associated requestHandler. The only APIs that cannot use this approach, and must be registered explicitly in CoreContainer.load are those v2 APIs that don't have a v1/requestHandler equivalent.

Solution

With this in mind, I'm removing my ill-conceived attempt at an ApiRegistrar class, to simplify the overall picture for API registration.

Tests

N/A - change is a refactor only, so existing tests must continue to pass.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.