[website] Remove some unused image files

Summary:
Looks like I forgot to actually delete image files in D9223
Also remove unused "hello_world" while we're at it.

Test Plan:
Grepped for image file names to check that they are in fact unused
Built and ran site locally, had a look at site to see that it looks OK.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9291