Drop unused logos and other homepage cleanup

[chalin]

As a followup to #716:

• Delete unused logos
• Ensure that CNCF and projects links introduced in Homepage: OTel is now an incubating project #716 use target="_blank" rel="noopener"

[theletterf]

Do we still have unused logos around?

[chalin]

I think that we do, but I'll have to double check.