Cleanup duplicate getting-started docs

Summary: Cleaning up this near-duplicate blurb in developer-notes due to the change in D750.

Test Plan: Reading the docs.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, Mengerian

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D774