We had a couple tools (checkstyle and pmd) that assumed configuration

keys were present, and we were satisfying that assumption by hard-coding

keys in the Review Bot's default configuration.
This isn't entirely clean or future-proof, and we had no real reason to

do it. It inspired bad practices, and tied the core code to certain

tools.
Instead, those keys are now removed from the default configuration, and

the tools themselves are expected to conditionally check for the

presence of these keys.
New unit tests were added to check the resulting logic, with and without

configuration keys present.

Unit tests passed.