remove obsolete code from CPUThreadPoolExecutor.h
remove obsolete code from CPUThreadPoolExecutor.h

Summary:
Now that folly::Func is defined as a noexcept-movable type
(folly::Function<void()>), we do not need to define copy/move
constructors and assignments operators in order to pretend the
CPUTask type is noexcept-movable. Now, CPUTask is properly
noexcept-movable, and the compiler provided implementations are,
too.

Reviewed By: yfeldblum

Differential Revision: D3973984

fbshipit-source-id: 45276abfa45f69af3f13e85690eea8ca19593199

facebook/wangle,facebook/wangle,facebook/wangle