remove unused method

lib/models/state.js

@@ -474,28 +474,6 @@ class State extends StateRecord {
     // TODO
   }
 
-  /**
-   * Remove characters from a node by `key` between offsets.
-   *
-   * @param {String} key
-   * @param {Number} startOffset
-   * @param {Number} endOffset
-   * @return {State} state
-   */
-
-  removeCharacters(key, startOffset, endOffset) {
-    let state = this
-    let node = state.getNode(key)
-
-    const characters = node.characters.filterNot((char, i) => {
-      return startOffset <= i && i < endOffset
-    })
-
-    node = node.merge({ characters })
-    state = state.updateNode(node)
-    return state
-  }
-
   /**
    * Split at a `selection`.
    *