[backport#16677] gui: remove unused PlatformStyle::TextColorIcon

Summary:
bca388db0da3d40953ff65d03da62ca45a7951a6 gui: remove unused PlatformStyle::TextColorIcon (fanquake)

Pull request description:

This is unused after #16612.

Depends on D8748

Backport of Core PR16677

Test Plan:

ninja all

run bitcoin-qt, check everything looks right

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8749