[Cashtab] Do not generate sourcemaps at npm start

Summary:
Depends on D15258

npm start will list warnings if dependency sourcemaps cannot be found. Many dependencies do not have sourcemaps so these warnings are not useful.

Also seems to load much faster this way.

Test Plan: npm start and no warnings about sourcemaps

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15259