Remove a use of String16::remove.

This function, ironically, is being removed. This is one of only two
users, and it's pretty hard to use correctly. In this case, using one
of the existing constructors to keep a prefix seems clearer even in a
world where remove() was better implemented.

Test: treehugger
Change-Id: Icdf02d9fcc059c396c13787d9bb4e8bda5658656