[Cashtab] Delete all functions, files, and mocks related to legacy tx building

Summary:
Cashtab now creates all txs using ecash-coinselect and unit-tested targetOutput building functions.

The legacy functions were tied together in many unit tests making it impractical to delete them in line with progress on the upgraded methods.

Now, though, they aren't used anywhere.

Test Plan:
npm test
arc lint confirms these deleted functions aren't called anywhere else, plus deleted mocks not used anywhere else

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15540