bpf: Remove unused MT_ENTRY define

The range tree introduction removed the need for maple tree usage
but missed removing the MT_ENTRY defined value that was used to
mark maple tree allocated entries.
Remove the MT_ENTRY define.

Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Link: https://lore.kernel.org/r/20241223115901.14207-1-lpieralisi@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Lorenzo Pieralisi

kernel/bpf/arena.c

@@ -257,8 +257,6 @@ static void arena_vm_close(struct vm_area_struct *vma)
 	kfree(vml);
 }
 
-#define MT_ENTRY ((void *)&arena_map_ops) /* unused. has to be valid pointer */
-
 static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 {
 	struct bpf_map *map = vmf->vma->vm_file->private_data;