[qt] coincontrol: Remove unused qt4 workaround

Summary:
This reverts 55eade9d46ebfa4b32b79893595d91d529fa30bb since it is no longer required.

Backport of Bitcoin Core PR13536
https://github.com/bitcoin/bitcoin/pull/13536

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4065