Add ClearRegisteredBenchmark() function. (#402)

* Add ClearRegisteredBenchmark() function.

Since benchmarks can be registered at runtime using the RegisterBenchmark(...)
functions, it makes sense to have a ClearRegisteredBenchmarks() function too,
that can be used at runtime to clear the currently registered benchmark and
re-register an entirely new set.

This allows users to run a set of registered benchmarks, get the output using
a custom reporter, and then clear and re-register new benchmarks based on the
previous results.

This fixes issue #400, at least partially.

* Remove unused change