[website] Remove ecashpage div ID

Summary:
The "ecashpage" ID is undefined, and doesn't seem to do anything.
Remove it in preparation for renaming the various "bcha" tags to "ecash" equivalents.

Test Plan: Built and ran it locally, it looks fine.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D10212