Remove custom fee radio group

Summary:

Removes the extraneous custom fee radio group and its single radio
button. The radio button is replaced with a label that has the
radio button's text.

This is a backport of Core PR11334

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

On send tab, select custom fee, verify that that "per kilobyte" is now a label instead of being a radio-button.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8177