get rid of hack to support label attribute for 1.7.0
get rid of hack to support label attribute for 1.7.0

Summary:
as titled, an ugly hack to support LABEL attribute for 1.7.0 which didn't exist.

getting rid of it now since oss support upgraded to 1.7.1

Differential Revision: D27536730

fbshipit-source-id: 58653e71cbf20a1cda0b7414f66b5f0014d89b84

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst