Remove gcc-v4.7 support from MemoryIdler
Remove gcc-v4.7 support from MemoryIdler

Summary:
[Folly] Remove gcc-v4.7 support from `MemoryIdler`.

As of gcc-v4.8, `std::is_trivial<std::chrono::duration<Rep, Period>>::value` holds. We no longer need any extra support.

Reviewed By: meyering

Differential Revision: D6673517

fbshipit-source-id: f28b2d126f113547ebf36c85f9fa37a5d944d65b

facebook/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly