[Cashtab] Remove unused function export

Summary:
T1746

This update function is not used outside of useWallet.js. Moreover, it is improperly defined (accepting the wrong param) based on an older version of the function.

Remove.

Test Plan: grep -r update src/*, verify function does not appear outside useWallet.js

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9890