[Cashtab] [Extension] [Code cleanup p1 retry] Remove unused variables from background.js

Summary:
T2751

Cashtab's extension codebase contains artifacts and some unused variables that were connected with the earliest versions, before getting it set up properly in the monorepo.

This stack goes through and cleans up background.js and contentscript.js

In this diff, unused variables are removed from background.js

Test Plan:
npm run extension
Build is good, works

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12543