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 for anything other than
clearing the entire string. In this case, assigning u"" seems clearer
even in a world where remove() was better implemented.

Test: treehugger
Change-Id: Ia6c7601f4303537a66bc3051e8a7743716367d6c