Checkstyle: Remove exceptions for JUnit 4 annotations
Checkstyle: Remove exceptions for JUnit 4 annotations

Checkstyle contains exceptions to allow certain exceptions in coding
style for JUnit 4 tests. For example, Checkstyle allows declaration of
a public variable if it is preceded by a Rule tag so as to ensure
correct variable assignment by JUnit 4.

After migrating all tests to JUnit 5, we no longer need these
exceptions for annotations specific to JUnit 4. Hence, we can remove
these annotations to tighten our strictness on coding style.

Hence, let's remove exceptions in checkstyle for JUnit 4 annotations.
Notably, let's remove
* Before/BeforeClass/After/AfterClass tags used to denote functions to
run before/after a test
* Rule/ClassRule tags used to denote rules (now extensions in JUnit 5)

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