[clang-format] Remove YAML hack to emit a BasedOnStyle comment (#89228)

When serializing a formatting style to YAML, we were emitting a comment
`# BasedOnStyle: <style>` if the serialized formatting style matches one
of the known styles. This is useful, but mis-uses the YAML API.

An upcoming change to fix keys with special characters by quoting them
breaks this,
and will emit a non-comment **key** `'# BasedOnStyle': <style>` instead.
(https://github.com/llvm/llvm-project/pull/88763)

Thus, remove this hack. There doesn't seem to be a specific use for it,
and it is not tested.

If we want the comment back, we should add comment support to the YAML writer,
and use that instead.