Simplified the logic in various functions.

Summary:
- Tidied up `ArcanistCSSLintLinter::getDefaultBinary`.
- Tidied up `CSSLintLinter::getDefaultFlags` function.
- Tidied up `ArcanistPhpcsLinter::getDefaultBinary` function.
- Tidied up `ArcanistPEP8Linter::getDefaultFlags` function
- Tidied up `ArcanistFlake8Linter::getDefaultFlags`.
- Tidied up `ArcanistCppcheckLinter::getLintOptions`.
- Tidied up `ArcanistCppcheckLinter::getLintPath`.
- Tidied up `ArcanistCpplintLinter::getLintOptions`.
- Tidied up `ArcanistCpplintLinter::getLintPath`.
- Removed child functions which are identical to the corresponding parent functions.

Test Plan: `arc lint` and `arc unit`.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: chad, Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8385