(Wangle) Clean up move constructors

(Wangle) Clean up move constructors

Summary:
These were doing more assignments than necessary.
1) We don't need to set `core_` to `nullptr` and then immediately
overwrite it with `other.core_`.
2) Don't need to swap the booleans (this was actually an unitialized
memory access).

Test Plan: Ran all the tests.

Reviewed By: hans@fb.com

Subscribers: trunkagent, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1852386

Signature: t1:1852386:1424375154:25997209e76ccd16169125597e868932a5143ffb