chore: Remove unused isFocusable*() functions.

These were needed in previous versions of plugin changes, but aren't
anymore.

core/interfaces/i_focusable_node.ts

@@ -60,20 +60,3 @@ export interface IFocusableNode {
    */
   onNodeBlur(): void;
 }
-
-/**
- * Determines whether the provided object fulfills the contract of
- * IFocusableNode.
- *
- * @param object The object to test.
- * @returns Whether the provided object can be used as an IFocusableNode.
- */
-export function isFocusableNode(object: any | null): object is IFocusableNode {
-  return (
-    object &&
-    'getFocusableElement' in object &&
-    'getFocusableTree' in object &&
-    'onNodeFocus' in object &&
-    'onNodeBlur' in object
-  );
-}

core/interfaces/i_focusable_tree.ts

@@ -119,22 +119,3 @@ export interface IFocusableTree {
    */
   onTreeBlur(nextTree: IFocusableTree | null): void;
 }
-
-/**
- * Determines whether the provided object fulfills the contract of
- * IFocusableTree.
- *
- * @param object The object to test.
- * @returns Whether the provided object can be used as an IFocusableTree.
- */
-export function isFocusableTree(object: any | null): object is IFocusableTree {
-  return (
-    object &&
-    'getRootFocusableNode' in object &&
-    'getRestoredFocusableNode' in object &&
-    'getNestedTrees' in object &&
-    'lookUpFocusableNode' in object &&
-    'onTreeFocus' in object &&
-    'onTreeBlur' in object
-  );
-}