Remove old structure time integration

[maxfirmbach]

What is currently missing in the new structure time integration to be finally be able to delete the old one? We start to drag along more an more old code that is not used anymore because many features are already available in the new one.

• Monolithic FSI
• ??

Are there more points to add?

@georghammerl @mayrmt

[amgebauer]

• Model order reduction with proper orthogonal decomposition (almost there)