Remove empty header and unnecessary includes.

The CUDA-specific binary search implementation has been `#ifdef 0`d
for a long time. It didn't perform as well as the sequential
implementation and is dead code that uses old dispatch mechanisms.

Also remove a load of unused headers from
`thrust/system/cuda/execution_policy.h`. The comments around these
headers don't make sense and looks like this was being used for test
bookkeeping.