.gitignore: ignore Doxygen database
drop unneeded doc/diagram/.gitignore
Drop thirdparty JSON implementations/performance tests

With the implementation of the more elaborate benchmarking
suite at https://github.com/miloyip/nativejson-benchmark,
the thirdparty performance tests are superseded.
performance.md: reference new benchmark suite
Merge pull request #180 from pah/cleanup/doxygen-thirdparty

Drop thirdparty libraries and minor cleanups