[website] Remove sponsors from website

Summary: This section is obsolete, the sponsors have been on there far longer than was promised so fine to remove them now.

Test Plan:
Ran it locally, looks ok
Check against commit 23f28bfc35f77ab341a27158ea6aa690e0e24383 to make sure it's removing everything it should

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11936