Clean up sponsor section of homepage

Summary:
Cleaning up the sponsor section of landing page

• removing reference to the fund.bitcoinabc.org page
• updated images to latests
• cleaned up styles

Test Plan: Ran locally, works, will check live site

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8581