[Cashtab] Remove testnet instantiations and unit tests

Summary: As per TG discussion, removing the redundant testnet environment instantiations and testnet specific unit tests.

Test Plan:

• npm test and ensure existing test suites all pass
• npm start
• grep -r 'testnet' src/ and ensure the only results are the 'testnet: false' attribute in mockTxBuilderObj.js
• create a new wallet (to trigger getWalletDetails) and ensure the successful creation of the wallet
• send a normal XEC tx to this new wallet's receiving address and ensure successful tx

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12411