[CMSSW_5_3_X] Remove dastest.cern.ch from relval_steps.py

dastest.cern.ch instance was only intended for CMSSW_6_2_X to
gather additional information in DAS system in order to solve a few
issues. CMSSW_6_2_X and CMSSW_7_0_X already use production instance
of DAS.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>