Revert "contrib: harfbuff: build internal ragel if not found"

ragel is used internally to generate some files when they are changed. But we don't modify any source, we can just build harfbuzz without ragel. The option is even off by default.

When it's not enabled there is this warning:

You have to install ragel if you are going to develop HarfBuzz itself

But that's not our case.

This reverts commit 071a7581.

Ref. #29291 (closed)