Drop thirdparty libraries and minor cleanups

With the implementation of the more elaborate [JSON benchmarking suite] [1] at, the third-party performance tests are superseded.  This pull-request drops the packaged copies of
 * JsonCpp
 * UltraJSON
 * Yail

and removes the performance tests from `perftest.h` and `premake4.lua`.

Additionally, an unused `.gitignore` is dropped and the generated Doxygen database file is ignored in the top-level `.gitignore`.

[1]: https://github.com/miloyip/nativejson-benchmark