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

@@ -118,7 +118,6 @@ exports.runMocha = (mocha, options) => {
   const {
     watch = false,
     extension = [],
-    grep = '',
     ui = 'bdd',
     exit = false,
     ignore = [],
@@ -138,7 +137,7 @@ exports.runMocha = (mocha, options) => {
   };
 
   if (watch) {
-    watchRun(mocha, {ui, grep}, fileCollectParams);
+    watchRun(mocha, {ui}, fileCollectParams);
   } else {
     exports.singleRun(mocha, {exit}, fileCollectParams);
   }

lib/cli/watch-run.js

@@ -16,14 +16,13 @@ const collectFiles = require('./collect-files');
  * Run Mocha in "watch" mode
  * @param {Mocha} mocha - Mocha instance
  * @param {Object} opts - Options
- * @param {string|RegExp} opts.grep - Grep for test titles
  * @param {string} opts.ui - User interface
  * @param {Object} fileCollectParams - Parameters that control test
  *   file collection. See `lib/cli/collect-files.js`.
  * @param {string[]} fileCollectParams.extension - List of extensions to watch
  * @private
  */
-module.exports = (mocha, {grep, ui}, fileCollectParams) => {
+module.exports = (mocha, {ui}, fileCollectParams) => {
   let runner;
   const files = collectFiles(fileCollectParams);
 
@@ -64,9 +63,6 @@ module.exports = (mocha, {grep, ui}, fileCollectParams) => {
   const rerun = () => {
     purge();
     eraseLine();
-    if (!grep) {
-      mocha.grep(null);
-    }
     mocha.suite = mocha.suite.clone();
     mocha.suite.ctx = new Context();
     mocha.ui(ui);