[Cashtab] Remove unused function export
[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

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