Use base browsers variable in Firestore karma config

Author: dlarocque

packages/firestore/karma.conf.js

@@ -21,7 +21,6 @@ const { argv } = require('yargs');
 module.exports = function (config) {
   const karmaConfig = {
     ...karmaBase,
-    browsers: getTestBrowsers(argv),
     // files to load into karma
     files: getTestFiles(argv),
 
@@ -76,12 +75,4 @@ function getTestFiles(argv) {
   }
 }
 
-function getTestBrowsers(argv) {
-  let browsers = ['ChromeHeadless'];
-  if (process.env?.BROWSERS && argv.unit) {
-    browsers = process.env?.BROWSERS?.split(',');
-  }
-  return browsers;
-}
-
 module.exports.files = getTestFiles(argv);