Remove String16::remove.

This function, ironically, is being removed.

Even more amusing, it was never "remove" anyway --- it literally did
the opposite, and removed everything *except* the range you passed to
it, and should probably have been called "keep"!

I'm looking at reimplementing much of libutils, but first I'm improving
test coverage, and literally every test I wrote for this failed. And
then when I fixed the "obvious bugs" in the implementation, I found
there actually were a couple of existing unit tests --- that mostly
served to demonstrate just how counter-intuitive this function was.

Bug: http://b/156999009
Test: treehugger
Change-Id: I41fd85f7c0988070f4039f607d2e57523d862ed9