Fix additional stale reference to Google usage

[charlesboyd]

Follow up to https://github.com/gwtproject/gwt-site/commit/5a32e0b7e6d219b82003ec6a6240dffca6e9ee1c.

GWT is no longer used by AdWords.

[niloc132]

Perhaps update it instead to point at gmail, since gmail now has gwt's jre emulation in it?

Last I checked, blogger and groups also still used GWT, are you sure all of these should be removed?

[niloc132]

Confirmed-ish by peeking at the optimized JS that the following definitely still use at least GWT's JRE, if not GWT itself:

• gmail
• wallet/pay
• groups (may be phasing out, haven't confirmed the new one either way, but have had to revert to the old one so that the page still works consistently)
• sheets
• app script
• blogger admin (may be phasing out, haven't confirmed or tried the new one)

It is hard to prove a negative, but we did get confirmation from Google once that AdWords no longer uses GWT.