Remove utils.FormatTimestampWithTZ

Long story short: time.strftime("%Z", time.localtime()) doesn't work,
even though it's documented to be equivalent to time.strftime("%Z").

$ TZ=America/Sao_Paulo python -c 'import time; print
time.strftime("%Z"), time.strftime("%Z", time.localtime())'
BRST LMT

References:
http://bugs.python.org/issue762963
https://bugs.launchpad.net/ubuntu/+source/python2.6/+bug/564607
http://stackoverflow.com/questions/4367896/issue-with-timezone-with-time-strftime



Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>