Remove unnecessary OAuth code

The parameters provided to the API endpoints only contain the
parameters specified in the method signature so there’s no need to
strip out the OAuth params.
Allow query string fallback for REST API SSL auth

In some environments, the PHP_AUTH_USER/PW server vars are empty which
prevents SSL authentication from working properly. This commit allows
the use of a query string fallback (e.g.
`?consumer_key=123&consumer_secret=abc`) for providing credentials over
SSL.

Fixes #5207
Merge pull request #5277 from maxrice/rest-api-fix-5207

Allow query string fallback for REST API SSL authentication