[Cashtab] Deprecate the auto-open camera on send feature

Summary:
The intended use case here of "it's common for people to send to a QR code most of the time they open the mobile app" has not really materialized. So, we are seeing more issues than benefit.

Deprecate the feature. Do not modify the default settings object, as we may wish to bring this back at a later date, and keeping one extra key in storage is nbd.

The "isMobile" function is no longer used in the app. However, there are no dependencies in it, and imo we could reasonably expect to use this function soon. Imo worth keeping in the codebase.

Test Plan:
npm test

grep -r loadWithScannerOpen src/ and no output

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17594