Update link to download AIR, remove outdated content

Resolves #5132. Thanks to @carljbowman for support

src/views/download/scratch2/download.jsx

@@ -113,9 +113,6 @@ class Download extends React.Component {
                         id="installation"
                     >
                         <div className="inner">
-                            <p className="callout">
-                                <FormattedHTMLMessage id="download.introMac" />
-                            </p>
                             <FlexRow className="three-col-row">
                                 <div className="installation-column">
                                     <div className="installation-column-number">
@@ -123,26 +120,6 @@ class Download extends React.Component {
                                     </div>
                                     <h3><FormattedMessage id="download.airTitle" /></h3>
                                     <p><FormattedHTMLMessage id="download.airBody" /></p>
-                                    <ul className="installation-downloads">
-                                        <li className="installation-downloads-item">
-                                            <FormattedMessage id="download.macOSX" /> -
-                                            {' '}<a href="http://get.adobe.com/air/">
-                                                <FormattedMessage id="download.download" />
-                                            </a>
-                                        </li>
-                                        <li className="installation-downloads-item">
-                                            <FormattedMessage id="download.macOlder" /> -
-                                            {' '}<a href="http://airdownload.adobe.com/air/mac/download/2.6/AdobeAIR.zip">
-                                                <FormattedMessage id="download.download" />
-                                            </a>
-                                        </li>
-                                        <li className="installation-downloads-item">
-                                            <FormattedMessage id="download.windows" /> -
-                                            {' '}<a href="http://get.adobe.com/air/">
-                                                <FormattedMessage id="download.download" />
-                                            </a>
-                                        </li>
-                                    </ul>
                                 </div>
                                 <div className="installation-column">
                                     <div className="installation-column-number">

src/views/download/scratch2/l10n.json

 {
   "download.title": "Scratch 2.0 Offline Editor",
   "download.intro": "You can install the Scratch 2.0 editor to work on projects without an internet connection. This version will work on Windows and MacOS.",
-  "download.introMac": "<b>Note for Mac Users:</b> the latest version of Scratch 2.0 Offline requires Adobe AIR 20. To upgrade to Adobe AIR 20 manually, go <a href=\"https://get.adobe.com/air/\">here</a>.",
   "download.installation": "Installation",
   "download.airTitle": "Adobe AIR",
-  "download.airBody": "If you don't already have it, download and install the latest <a href=\"http://get.adobe.com/air/\">Adobe AIR</a>",
+  "download.airBody": "If you don't already have it, download and install the latest <a href=\"https://airsdk.harman.com/runtime\">Adobe AIR</a>",
   "download.macOSX": "Mac OS X",
   "download.macOlder": "Mac OS 10.5 & Older",
   "download.windows": "Windows",