Remove `X-XSS-Protection` grading entirely.

[lgarron]

This addresses https://github.com/mozilla/http-observatory-website/issues/254

To quote https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection :

Warning:

Even though this feature can protect users of older web browsers that don't yet support CSP, in some cases, XSS protection can create XSS vulnerabilities in otherwise safe websites. See the section below for more information.

Note:

• Chrome has removed their XSS Auditor
• Firefox has not, and will not implement X-XSS-Protection
• Edge has retired their XSS filter

The cited "section below" provides a concrete example of how the XSS filter can be harmful: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection#vulnerabilities_caused_by_xss_filtering

This PR is a very simple strawperson. It might be worth to select a more "informational" alternative:

• Recognizing possible header values without giving them a score.
• Recognizing certain header values as reasonable, such as block.
• Not grading the header, but putting some sort of notice if the header was observed, e.g. "The Mozilla TLS Observatory used to grade the X-XSS-Protection header, but this is no longer the case. For details, see: [link here]"

[LeoMcA]

This is being handled with https://github.com/mozilla/http-observatory/pull/520, thanks!