Remove the `spotlessDiagnose` task from the gradle plugin

Some formatters have idempotency bugs. Spotless has a [padded cell](https://github.com/diffplug/spotless/blob/main/PADDEDCELL.md) feature which can fix them.

A long time ago, this feature was off by default. A user would get an error related to idempotency, they could run `spotlessDiagnose` to understand it, and then they might turn on the padded cell feature.

In [`3.29.0` released May 2020](https://github.com/diffplug/spotless/blob/main/plugin-gradle/CHANGES.md#3290---2020-05-05) we made padded cell mandatory, so idempotency problems are always silently fixed.

The maven plugin has never had a "diagnose" task. Probably no one would notice if we removed it from the Gradle plugin also.

Here is the code:

https://github.com/diffplug/spotless/blob/88c83de2450b8fad8174ff08a5c95fa18a915617/plugin-gradle/src/main/java/com/diffplug/gradle/spotless/SpotlessDiagnoseTask.java#L34-L77

And here is info on some tech debt that it has accumulated. Probably better to just delete it than fix it. Anybody object?

- https://github.com/diffplug/spotless/pull/2719#discussion_r2525334770