Merge #12779: Qt: Remove unused method setupAmountWidget(...)

Summary:
3a0f8d7 Qt: Remove unused method setupAmountWidget(...) (practicalswift)

Pull request description:

Remove unused method `setupAmountWidget(...)`.

Last use removed in f193c57a63d8e66835873ff05ef8028fa87b427f.

Tree-SHA512: 8f25ed2da13cfbc0f2e042286b3c979ef03de960028d7824110bdc2ed96f5075e3b35a9809a09c83caa5bad2237d2e048ff212b19a0be1d07f79c19691dfcd87

Backport of Core PR12779
https://github.com/bitcoin/bitcoin/pull/12779/

Test Plan:

make check
./bitcoin-qt -> send and receive transactions, perform some console rpc commands, and search transactions

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3899