Don't re-initialize grep option on watch re-run

We remove code that called `mocha.grep(null)` on watch re-runs if the
`--grep` option was not supplied. The code seems to serve no purpose and
is the cause of #2027.

lib/cli/run-helpers.js

@@ -188,19 +188,18 @@ exports.singleRun = (mocha, {files = [], exit = false} = {}) => {
  * @param {Object} [opts] - Options
  * @param {boolean} [opts.watch=false] - Enable watch mode
  * @param {string[]} [opts.extension] - List of extensions to watch
- * @param {string|RegExp} [opts.grep] - Grep for test titles
  * @param {string} [opts.ui=bdd] - User interface
  * @param {boolean} [opts.exit=false] - Force-exit Mocha when tests done
  * @param {string[]} [files] - Array of test files
  * @private
  */
 exports.runMocha = (
   mocha,
-  {watch = false, extension = [], grep = '', ui = 'bdd', exit = false} = {},
+  {watch = false, extension = [], ui = 'bdd', exit = false} = {},
   files = []
 ) => {
   if (watch) {
-    watchRun(mocha, {extension, grep, ui, files});
+    watchRun(mocha, {extension, ui, files});
   } else {
     exports.singleRun(mocha, {files, exit});
   }

lib/cli/watch-run.js

@@ -16,12 +16,11 @@ const Mocha = require('../mocha');
  * @param {Mocha} mocha - Mocha instance
  * @param {Object} opts - Options
  * @param {string[]} opts.extension - List of extensions to watch
- * @param {string|RegExp} opts.grep - Grep for test titles
  * @param {string} opts.ui - User interface
  * @param {string[]} opts.files - Array of test files
  * @private
  */
-module.exports = (mocha, {extension, grep, ui, files}) => {
+module.exports = (mocha, {extension, ui, files}) => {
   let runner;
 
   console.log();
@@ -61,9 +60,6 @@ module.exports = (mocha, {extension, grep, ui, files}) => {
   const rerun = () => {
     purge();
     eraseLine();
-    if (!grep) {
-      mocha.grep(null);
-    }
     mocha.suite = mocha.suite.clone();
     mocha.suite.ctx = new Context();
     mocha.ui(ui);