[OneStepGridOperator] Remove unused class OneStepLocalJacobianResidualAssemblerEngine

For some reason, there are two different engine implementations for
concurrently assembling residuals and jacobian in an explicit
time-stepping scheme. However, only the class
OneStepExplicitLocalJacobianResidualAssemblerEngine is used anywhere in
PDELab, so this patch removes the other one to avoid unnecessary confusion.
[GridOperator] Make local matrix writable in utility method etadd_symmetric()

etadd_symmetric() has to modify the local matrix in order to clear out
constrained columns, but the object is passed in by const reference.

Fixed.