path: remove dead code in favor of unit tests

Remove dead code paths that are created by assertions that will never
trigger. They may only trigger if either the `splitDeviceRe` or
`splitPathRe` regular expressions are modified. If at some point they
are modified, current unit tests will catch most of the resulting
errors and this commit adds extra tests to catch the remaining errors.

PR-URL: https://github.com/nodejs/io.js/pull/2282


Reviewed-By: Roman Reiss <me@silverwind.io>