Drop WTF_DISALLOW_CONSTRUCTION_FROM_ZERO and WTF_DISALLOW_ZERO_ASSIGNMENT

Those macros had been added to avoid inconsistency between native and
home-brewed versions std::nullptr_t. Now we have only native
std::nullptr_t so there is no reason to keep them.

Review URL: https://codereview.chromium.org/687003004

git-svn-id: svn://svn.chromium.org/blink/trunk@184597 bbb929c8-8fbe-4397-9dbb-9b2b20218538