D24960: Remove call to no longer needed KIconTheme::assignIconsToContextMenu

[Volker Krause]

vkrause added a task: T11865: Deprecate KIconTheme::assignIconsToContextMenu.

REPOSITORY
  R310 KTextWidgets

REVISION DETAIL
  https://phabricator.kde.org/D24960

To: vkrause
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns