Remove String16::makeLower().

If you need to do a case transformation for a Unicode string, you need
to use icu4c. This only worked for ASCII, which is just silly. Luckily
it doesn't seem to be used anywhere.

Test: treehugger
Change-Id: I4a864823ec35a0b57b50909587cc3efac3f531a7
Merged-In: I4a864823ec35a0b57b50909587cc3efac3f531a7
(cherry picked from commit c74d97c8)