btree/node.rs: remove incorrect comment from pop_internal_level docs

Author: btj

library/alloc/src/collections/btree/node.rs

@@ -600,9 +600,6 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::LeafOrInternal> {
     /// no cleanup is done on any of the keys, values and other children.
     /// This decreases the height by 1 and is the opposite of `push_internal_level`.
     ///
-    /// Requires exclusive access to the `NodeRef` object but not to the root node;
-    /// it will not invalidate other handles or references to the root node.
-    ///
     /// Panics if there is no internal level, i.e., if the root node is a leaf.
     pub(super) fn pop_internal_level<A: Allocator + Clone>(&mut self, alloc: A) {
         assert!(self.height > 0);