No need to std::move local variable to return value

- when no std::move is used, local variable that is returned is directly
  constructed in the address of the return value
- when std::move is used, the return value has to be move constructed
  from the rvalue of std::move, resulting in one more ctor call, not to
  mention the added code complexity
- See Return Value Optimization in Effective Modern C++ by Scott Meyer,
  Item 25

- also delete unrelated outdated comment

Test: compiles and runs
Change-Id: I1b71ea01c6ab1788432e846d9feeb2df608e6b6d
Signed-off-by: Daniel Zhang <danielzhang130@gmail.com>