Don't typedef types to the same name in different namespaces

Don't typedef types to the same name in different namespaces

Summary:
This was causing MSVC to complain due to `DSched` already being declared in `BatonTestHelpers.h` as `::folly::DSched`, but it's then typedef'd again in `BatonTest.cpp`, this time as `::DSched`. MSVC complains about the duplicate definitions even though they resolve to the same type, so kill the one in the source file and keep the one in the header.
The same is the case with `GroupVarint32Decoder` and `GroupVarint64Decoder`, which are both declared in `GroupVarint.h` and then again, this time in an anon namespace, in `GroupVarintTest.cpp`.

Reviewed By: yfeldblum

Differential Revision: D3600948

fbshipit-source-id: 401790aa918289fe4a34dd5af683f8c6ca50d847