[Cashtab] Removed obsolete dev note from Token.js

Summary: Removed obsolete dev note from Token.js, related to task T2259.

Test Plan: navigate to Token.js, ensure obsolete dev note has been removed.

Reviewers: bytesofman, #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11074