Don't need swap anymore.

The declaration of std::swap here is inconsistent with the one in
libc++ (noexcept differences). Rather than worrying about keeping them
all consistent, just remove swap. It was only ever added for ART, and
ART now uses std::unique_ptr instead.

This was causing an ambiguous overload error in system/core.

Change-Id: I4cf4c41c5dda01a96915e8309af2f8be089d1a2a