Build: remove travis.yml and travis mentions from core (#4983)

We've migrated to GH actions on jQuery core and have already disabled travis builds for core.

.npmignore

@@ -4,7 +4,6 @@
 /.editorconfig
 /.gitattributes
 /.mailmap
-/.travis.yml
 
 /build
 /external

.travis.yml

@@ -1,56 +0,0 @@
-language: node_js
-os: linux
-node_js:
-- "10" # required by jQuery infra
-- "12"
-- "14"
-- "16"
-env:
-  - NPM_SCRIPT=test:browserless
-jobs:
-  include:
-    - name: "Browser tests: full build, Chrome & Firefox stable"
-      node_js: "14"
-      env:
-        - NPM_SCRIPT="test:browser"
-        - BROWSERS="ChromeHeadless,FirefoxHeadless"
-      addons:
-        chrome: stable
-        firefox: latest
-    - name: "Browser tests: slim build, Chrome stable"
-      node_js: "14"
-      env:
-        - NPM_SCRIPT="test:slim"
-        - BROWSERS="ChromeHeadless"
-      addons:
-        chrome: stable
-    - name: "Browser tests: no-deprecated build, Chrome stable"
-      node_js: "14"
-      env:
-        - NPM_SCRIPT="test:no-deprecated"
-        - BROWSERS="ChromeHeadless"
-      addons:
-        chrome: stable
-    - name: "Browser tests: ES modules build, Chrome stable"
-      node_js: "14"
-      env:
-        - NPM_SCRIPT="test:esmodules"
-        - BROWSERS="ChromeHeadless"
-      addons:
-        chrome: stable
-    - name: "Browser tests: AMD build, Chrome stable"
-      node_js: "14"
-      env:
-        - NPM_SCRIPT="test:amd"
-        - BROWSERS="ChromeHeadless"
-      addons:
-        chrome: stable
-    - name: "Browser tests: full build, Firefox ESR"
-      node_js: "14"
-      env:
-        - NPM_SCRIPT="test:browser"
-        - BROWSERS="FirefoxHeadless"
-      addons:
-        firefox: latest-esr
-script:
-  - npm run $NPM_SCRIPT

Gruntfile.js

@@ -22,7 +22,7 @@ module.exports = function( grunt ) {
 	var fs = require( "fs" ),
 		gzip = require( "gzip-js" ),
 		oldNode = /^v10\./.test( process.version ),
-		isCi = process.env.TRAVIS || process.env.GITHUB_ACTION,
+		isCi = process.env.GITHUB_ACTION,
 		ciBrowsers = process.env.BROWSERS && process.env.BROWSERS.split( "," );
 
 	if ( !grunt.option( "filename" ) ) {