Allow save_post_meta to delete single meta elements instead of update

At that stage in the code we don't know if there are multiple values in the DB, so the best course of action is to just deal with it like there are multiple elements.

Fixes #2213
Handle user/comment multi simple meta issues
Merge pull request #2363 from jamesgol/simplemulti

Allow save_post_meta to delete single meta elements instead of update