Remove String16::remove tests.

These tests are only in internal right now, preventing me from removing
this function entirely. So let's get them out of the way...

Test: treehugger
Ignore-AOSP-First: workaround for presubmit merge conflict
Bug: http://b/35267372
Change-Id: Ib5de178ffad833c87e8e93bfd33e85a0ff504ac0