[clangd] Cleanup FuzzyFindRequest serialization and dex benchmark

* Due to the LLVM's JSON library changes (?), FuzzyFindRequest serialization is
  no longer valid since arrays are serialized as llvm::json::Array already.
  Hence, current implementation creates a nested array.
* YAML format is no longer the default, mention this for the benchmark.
* FIXME is no longer relevant. I ran benchmarks that showed no improvement with
  priority_queue years ago.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D106432