No CMake option to disable building microbenchmark

[dakrk]

I'm using libFLAC as a Git submodule in a project, and I've noticed that it's building a benchmark_residual binary with no way to disable that. I would like it if there was, as it's pointless for everything to always build it.

(I would've done it but I wasn't sure if it should just follow the BUILD_TESTING option because it's outside of the test directory and it's just a benchmark, or if there would be a BUILD_BENCHMARK option instead)

[ktmf01]

I don't think that benchmark is being used anymore, perhaps best to simply remove it.