Stop using PMD and CheckStyle for quality controls

[alexander-yevsyukov]

We decided to gradually migrate our codebase to Kotlin.
ErrorProne and detekt cover majority of quality issues we need to spot in our Java and Kotlin code.
Configuring and maintaining PMD and CheckStyle takes build and developers' time.

Let's standardize on ErrorProne and detekt and drop other code quality checks.

[alexander-yevsyukov]

@armiol, what would you say?

[armiol]

@alexander-yevsyukov let's discuss vocally. I say we remove both PMD and Checkstyle, but there are a few details to consider.