[BOLT][NFC] Simplify YAMLProfileWriter::convert

Use `getAnnotationWithDefault` instead of testing if the annotation is
set. If the default value is used, and `CSI.Count` is set to zero, the
target is discarded by a check below.

Test Plan: NFC

Reviewers: maksfb, dcci, rafaelauler, ayermolo

Reviewed By: ayermolo

Pull Request: https://github.com/llvm/llvm-project/pull/82129