Fix: Remove default .js from --ext CLI option (#13176)

PR #12677, `Breaking: lint overrides files`, removed the default `.js`
value from the  default CLIEngine options. However, it did not remove
the same default value from the `--ext` option parsed by Optionator.
Because of this, when running `eslint .` without the `--ext` option,
Optionator would insert a default `--ext .js` value that would be passed
on to the CLIEngine. ESLint only lints `overrides` files if the `--ext`
CLI option is not passed, so the default `--ext .js` option prevented
ESLint from ever reaching the `overrides` file path checking flow.

lib/options.js

@@ -46,7 +46,6 @@ module.exports = optionator({
         {
             option: "ext",
             type: "[String]",
-            default: ".js",
             description: "Specify JavaScript file extensions"
         },
         {

tests/lib/options.js

@@ -79,11 +79,10 @@ describe("options", () => {
             assert.strictEqual(currentOptions.ext[1], ".js");
         });
 
-        it("should return an array one item when not passed", () => {
+        it("should not exist when not passed", () => {
             const currentOptions = options.parse("");
 
-            assert.isArray(currentOptions.ext);
-            assert.strictEqual(currentOptions.ext[0], ".js");
+            assert.notProperty(currentOptions, "ext");
         });
     });