Remove redundant subtab title

[ericlaw1979]

Vertical space is precious, and Fiddler has too many nested tabs already.

[vcsjones]

First: I am horrible at UX/UI.
Second: there is another tab beside certificates when the session had HPKP headers.

I don't think that's the best way to do this, but that explains why there is another tab row. Since HPKP has more to do with the HTTP session than the certificate collection, I felt it needed to be somehow not attached to the certificates.

I suppose I can just not show the tabs at all if there is only a single tab, or move the HPKP header info somewhere else.