We had to force-include Char16.h to simulate char16_t on older MSVC versions.
But it is no longer the case. We should not rebuild the world whenever we
touch this file.
MozReview-Commit-ID: 1XY7tQD8LoK
We don't need the tiny helper function for single-character assignment,
and the multi-argument assign() method is the only thing that uses it,
which is also itself unused. So remove them both.
