[heap] Cleanup string factory methods a bit.

[heap] Cleanup string factory methods a bit.

This is mostly removing dead code and also dropping MUST_USE_RESULT
annotations from methods that cannot throw an exception anyways.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1370153002

Cr-Commit-Position: refs/heads/master@{#30995}