[ecash-lib] Remove initWasm

Summary:
This is a big inconvenience for users, instead we import it as base64 into the bundle directly.

In the future, we can also remove the requirement to thread in Ecc, and just instantiate an instance globally.

Test Plan: npm test, also publish the library as an RC and see if it works in the browser

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17639