[Cashtab] [final countdown bch param p2] Delete getRestUrl function

Summary:
T2730

Depends on D12805

Delete the getRestUrl function

Delete all of its calls.

Some of its calls relate to error messages only known to exist in bch-api. In these cases, generalize the error msg.

Test Plan:
Review code and confirm deletions only relate to error notifications.
npm test
npm start
Send an eToken tx
Send an XEC tx
Create a token
Burn some of an eToken

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12807