sixDoFRigidBodyDisplacementPointPatchVectorField, uncoupledSixDoFRigidBodyDisplacementPointPatchVectorField: removed

These legacy boundary conditions are no longer needed and have been superseded
by the more flexible sixDoFRigidBodyMotion and rigidBodyMotion solvers.  See tutorials:

incompressible/pimpleDyMFoam/wingMotion/wingMotion2D_pimpleDyMFoam
multiphase/interDyMFoam/RAS/DTCHull
multiphase/interDyMFoam/RAS/floatingObject

Resolves bug-report https://bugs.openfoam.org/view.php?id=2487