Kill <meta name="keywords" content="..."> in a blazing fire.

meta-keywords is completely and totally useless. It has absolutely no effect
on search engine and no-one uses it anymore. Time to euthanize it.

There isn't a single user of OutputPage::addKeywords inside any extension
inside of Gerrit. So I'm just deleting this method without a deprecation.

Change-Id: I188755521dcde3a9e191975d1ae3cabf7a5d67cd
Merge "Kill <meta name="keywords" content="..."> in a blazing fire."