Remove dead code to fix test failure due to NoSuchMethodError

This CL removes following methods from StringUtils, which are
no longer used since Id3c262386a7bc7ed75966b1395a50171abe550d3.
- #joinCommaSplittableText
- #appendToCommaSplittableTextIfNotExists

BUG: 16000850
Change-Id: Ibdc9b7e463a19cdfac788b24c0a6322d0c5850c5