[e.cash] Remove instant exchange widgets

Summary:
As discussed removing the swapzone and swapspace widgets from /get-ecash page
Also removing the related css class and next config settings

Test Plan: preview and check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18922