Remove warning in GRHydro_InitData

[Zach Etienne]

An unusual warning popped up when trying to compile GRHydro_InitData a little while ago:

GRHydro_InitData/src/GRHydro_PoloidalMagFieldM.F90:110:2: warning: #warning "This algorithm does only work on Cartesian grids!!"

The assumption of Cartesian grids is so ingrained in Toolkit thorns that if we were consistent in this warning, most of the output from the compiler would be this warning.

[Roland Haas]

Fixed in git hash 8e3dda4 "GRHydro_InitData: check for curvilinear grid at runtime" of einsteinevolve