chore: delete src/MultiMap (#5690)

Went to migrate to TypeScript but a grep of the codebase for `MultiMap` reveals that nothing requires it :)

src/Multimap.js

@@ -1,136 +0,0 @@
-/**
- * Copyright 2017 Google Inc. All rights reserved.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-/**
- * @template T
- * @template V
- */
-class Multimap {
-  constructor() {
-    this._map = new Map();
-  }
-
-  /**
-   * @param {T} key
-   * @param {V} value
-   */
-  set(key, value) {
-    let set = this._map.get(key);
-    if (!set) {
-      set = new Set();
-      this._map.set(key, set);
-    }
-    set.add(value);
-  }
-
-  /**
-   * @param {T} key
-   * @return {!Set<V>}
-   */
-  get(key) {
-    let result = this._map.get(key);
-    if (!result)
-      result = new Set();
-    return result;
-  }
-
-  /**
-   * @param {T} key
-   * @return {boolean}
-   */
-  has(key) {
-    return this._map.has(key);
-  }
-
-  /**
-   * @param {T} key
-   * @param {V} value
-   * @return {boolean}
-   */
-  hasValue(key, value) {
-    const set = this._map.get(key);
-    if (!set)
-      return false;
-    return set.has(value);
-  }
-
-  /**
-   * @return {number}
-   */
-  get size() {
-    return this._map.size;
-  }
-
-  /**
-   * @param {T} key
-   * @param {V} value
-   * @return {boolean}
-   */
-  delete(key, value) {
-    const values = this.get(key);
-    const result = values.delete(value);
-    if (!values.size)
-      this._map.delete(key);
-    return result;
-  }
-
-  /**
-   * @param {T} key
-   */
-  deleteAll(key) {
-    this._map.delete(key);
-  }
-
-  /**
-   * @param {T} key
-   * @return {V}
-   */
-  firstValue(key) {
-    const set = this._map.get(key);
-    if (!set)
-      return null;
-    return set.values().next().value;
-  }
-
-  /**
-   * @return {T}
-   */
-  firstKey() {
-    return this._map.keys().next().value;
-  }
-
-  /**
-   * @return {!Array<V>}
-   */
-  valuesArray() {
-    const result = [];
-    for (const key of this._map.keys())
-      result.push(...Array.from(this._map.get(key).values()));
-    return result;
-  }
-
-  /**
-   * @return {!Array<T>}
-   */
-  keysArray() {
-    return Array.from(this._map.keys());
-  }
-
-  clear() {
-    this._map.clear();
-  }
-}
-
-module.exports = Multimap;