qt: Remove "Pay only required fee" checkbox

Summary:

The custom fee input box now has a minimum value equal to the minimum
required fee. Before a value below the minimum fee could be entered
which was confusing since the minimum fee would still be paid even
though a lower amount was entered.

Backport of Core PR14608 [2/2]
Commit https://github.com/bitcoin/bitcoin/pull/14608/commits/a16f44c040642432e234ad7317b00fe829c2d9e7

Depends on D8176

Test Plan:
ninja && src/qt/bitcoin-qt -regtest

On the Spend page, custom fee section, verify that the "Pay only required fee" checkbox is no longer there and is replaced by a simple label.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8178