Remove or replace two mutable data members from SteppingHelixPropagator

The class SteppingHelixPropagator has four mutable data members, all of which pose thread safety issues.
This pull request removes or replaces two of them.  One that was unused was removed. The other was easily replaced by a variable on the stack.
The two remaining mutable data members are more difficult to replace, and will be left alone for now.
Tested with checkdeps -a and the relval matrix.