[cleanup] Remove unused allocators

Remove allocator template parameter from the classes base_array_unmanaged, compressed_base_array_unmanaged, block_vector_unmanaged, compressed_block_vector_unmanaged, and CompressedBlockVectorWindow. All of these classes do not manage memory on their own. The provided allocator was only used to deduce size_type. The latter is now provided as template parameter explicitely.

Notice that this (seemigly breaking) change is safe, because all these classes are clearly marked as implementation details that should not be used outside of dune-istl.