Remove restrict/__restrict.

We've never really used __restrict: only <string.h> and <stdio.h> (which
are still very similar to upstream BSD headers) consistently have these
annotations. Neither clang nor GCC warns for trivial cases, and there's
little obvious documentation benefit.

Bug: http://b/30833514
Test: builds
Change-Id: I3e4384281865475d0c55d764b546d8166419ee31