Drop IE fallback from SecurityHeadersAttribute.cs sample file

[wenz]

The SecurityHeadersAttribute.cs sample file sets the X-Content-Security-Policy header that was used by ancient versions of Internet Explorer. I propose that this is dropped from the samples: IE is gone for good, and had only a very limited support of Content Security Policy anyway.

[wenz]

PR: DuendeSoftware/samples#279

[maartenba]

Thanks @wenz ! No need to raise a discussion (unless you do want to discuss). Looked at PR and looks great, and will merge soon. Much appreciated!