Drop unused special-case for string-valued heap entries
Drop unused special-case for string-valued heap entries

Summary:
In practice, Flow doesn't use string-valued heaps at all. Even if we did, the
non-string case will still work, just with an additional malloc/free and
serialization step, which is reasonably efficient as well.

My motivation here is mostly simplicity, but eliminating this case also brings
the existing shared heap closer to the new heap design I'm working towards.

Reviewed By: gabelevi

Differential Revision: D19267093

fbshipit-source-id: c5800400fff0c60c6c37ff7000b46162376af587

mroch/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,mroch/flow,mroch/flow