Summary:
Since we require C++14 in Folly and use `std::enable_if_t` in some places, there
is no need for the `_t` workaround. Replace `_t<enable_if<...>>` with
`enable_if_t<...>`.

Reviewed By: yfeldblum

Differential Revision: D13511564

fbshipit-source-id: 314b4a63281ce6b8275174ae89fab5fba1101bfb