Removed an unused variable in Momentum_Equation.F90.
Removed an unused variable in Momentum_Equation.F90.

rjferrier/fluidity,jjo31/ATHAM-Fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,jrper/fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,rjferrier/fluidity,iakovos-panourgias/fluidity,jjo31/ATHAM-Fluidity,rjferrier/fluidity,jjo31/ATHAM-Fluidity,jrper/fluidity,iakovos-panourgias/fluidity,iakovos-panourgias/fluidity