Removed steps in FatigableMuscle that manually add '_deriv' variables
Removed steps in FatigableMuscle that manually add '_deriv' variables

opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core