[Cashtab] Remove unused screenshots folder

Summary: Related to T2817. Screenshots are not used and they do not reflect the current iteration of the Cashtab app.

Test Plan:
cd web/cashtab && ls
observe that the screenshots folder is no longer present

Reviewers: bytesofman, emack, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12684