Remove unused methods from AbstractRewriteTask and RewriteClassLoader

[timtebeek]

What's your motivation?

We've had reports in #212 that parallel and partial runs throw an exception when trying to load duplicate classes.

Have you considered any alternatives or workarounds?

0. The docs on untracked are not a 100% clear on if this avoids all parallel runs.
1. There's an alternative in getOutputs().upToDateWhen(task -> false); which we could set in AbstractRewriteTask instead.
2. With more effort we might be able to tweak DelegatingProjectParser to support parallel classloaders each with their own GradleProjectParser, but the impact of that on reliable recipe runs is hard to judge at this moment, but likely leads to fragile runs.
3. We could try to avoid the duplicate class loading by adding locks on org.openrewrite.gradle.RewriteClassLoader#loadClass, but it's not clear whether that's a viable option here, and again could lead to fragile recipe runs.

Any additional context

• Accidentally pushed to main: 8942452, which then closes loader org.openrewrite.gradle.RewriteClassLoader attempted duplicate class definition #212

[shanman190]

@timtebeek, sadly this doesn't disable parallel.

@UntrackedTask indicates that there shoudn't be any optimizations reliant upon input/output calculations (incremental, build cache, etc).

[timtebeek]

Was already suspicious; who knew AI assistants could lie? 🙃

Among the other options there's also apparently something called shared build services, but that appears to be from 6.1 onwards, which isn't great with our focus to support back to 4.10. Any other suggestions perhaps?

[shanman190]

Java, by default, expects for loadClass to be thread safe. Ironically, even the parent implementation is thread safe, but our child implementation is not. All of the necessary plumbing is accessible to us though in order to make it thread safe, so I think we should go ahead and do that to resolve these parallel loading issues once and for all.

[timtebeek]

Ok I had intended to push your suggestion to this branch, but instead I had switched back to main before 8942452. 😞

Updated the PR description for the small adjustments we're left with here, which are now very much optional to get in.