Remove incompatible Unicode compatibility types.

As well intentioned as these were, uint16_t and C++11's char16_t are
_not_ actually compatible. They are not implicitly convertible, and
they mangle differently, so they are not even ABI compatible. In our
now wonderous world of C++11, no one should be using these, so just
kill them.

Bug: 18300613
Change-Id: I06d92d7f1d937dd94a620874323d4c50eb6a31bd