🍒 Revert "lookandfeel: Explicitely set isMask of ActionButton"

This reverts commit 2d46de67.

This change made sense to fix bug 474377 back when these icons came from the Plasma style. However subsequently that feature was removed. As a result, icons from the desktop icon theme are now shown here, and we can't guarantee their visual style. As such, forcing them to be a mask is now incorrect and obliterates the icon if it's using the full-color style.

BUG: 477995 CCBUG: 474377 FIXED-IN: 6.0

(cherry picked from commit 8fa333fb)