Remove custom JSON parsing

When the SDK was originally written, the decision was made to use custom JSON parsing for a few reasons:
-Some fields required custom handling (datetime fields, for example)
-Consistency with the Box Java SDK

After real-world use, it turns out that these reasons are inadequate to justify a custom JSON parser. Simply defaulting to the built in JSON class methods is a much better option both in terms of performance and usability.

No files or tests are named. Inspect the SDK's custom JSON parsing entry points and existing tests first, then compare their behavior with the built-in JSON class methods, including datetime handling. Done means the custom parser is removed and existing parsing behavior remains covered by the test suite.

Written by the indexing model from the issue text.