[Cashtab] [Extension] [Code cleanup p2 retry] Remove unused window variable from background.js

Summary:
T2751

Depends on D12543

The window is never used as a variable. Calling await openWindow does the same thing.

Test Plan:
npm run extension
Update your local unpacked extension
Navigate to components.cashtab.com
Click on Cashtab Pay button for the standard Cashtab Badge component
Cashtab extension should open with tx populated

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12544