drm/amdgpu: Don't call interval_tree_remove in amdgpu_mn_destroy

rbtree_postorder_for_each_entry_safe can skip over some entries if
the tree is rebalanced in interval_tree_remove. interval_tree_remove
is also redundant when the tree is just about to be freed.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>