[alugrid] Kill obsolete forward declarations for ALUSimplexGrid/ALUCubeGrid
alugrid.hh in 2.4 still had forward-declarations for ALUSimplexGrid and ALUCubeGrid which were never defined. Remove them.
Also take the opportunity to remove a duplicate
Note that this merge request is against 2.4, as alugrid.hh is gone from master.
WIP: Testing not possible since the relevant unit tests fail anyway. Good for merge due to simplicity. Leaving to the discretion of the 2.4.1 release manager (see #6 (closed)).