Remove user-defined move constructor and assignment operator from WTF::String

WTF::String can rely on implicit move constructor and assignment
operator as WTF::RefPtr class has them defined now (r177066).

Review URL: https://codereview.chromium.org/367343003

git-svn-id: svn://svn.chromium.org/blink/trunk@178541 bbb929c8-8fbe-4397-9dbb-9b2b20218538