Remove Code: Schlaffer BC

In VirtualFluids we have some unused code related to Schlaffer BC. Code related to Schlaffer can be found in VirtualFluids_GPU and is linked to this issue (search for issue url). When the code is removed, the corresponding commit will be added here. We will remove this code in an upcoming commit. This Issue is supposed to be a bookmark, so that the code can be restored if necessary.

• delete commit: 5c33cca3
• MR: https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/merge_requests/321
• last commit containing BCs: 944091df