Get rid of utils.CommaJoin

- We never remember to use it (5 uses vs 21 " ,".join())
- It's longer to write than " ,".join()
- The added value of the apostrophe in the string is not very much

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>