Remove sponsor signup page from HP
Remove sponsor signup page from HP

The prospectus link is already dead and PromCon removed from the main
CNCF events prospectus, so I assume sponsorships are closed.

Signed-off-by: Julius Volz <d7748b1e6f41fd74500c5ccae0980eab88c2ef61@gmail.com>

promcon/website,promcon/website,promcon/website,promcon/website,promcon/website,promcon/website