BUG: Remove unused QUASI_NEWTON_STEP_ERROR.

This StopCriterion that may have been intended for the QuasiNewton v4
Optimizer was never used.

Change-Id: Id00cbbc9dbd3a95b26ba7c2bbd6e15ae783dc09b
Merge topic 'QUASIStopCondition'

b7e1218 BUG: Remove unused QUASI_NEWTON_STEP_ERROR.