Remove unnecessary update()

Summary:
This `update()`, added in D11796, is no longer necessary since D11795
switched to using `QGraphicsOpacityEffect` instead of `setOpacity`.

Test Plan:
Open a raster and SVG image in {nav View} compare mode.
Zoom both images in and out, and ensure no artifacts like parts of the
selection rect are left over.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: muhlenpfordt

Differential Revision: https://phabricator.kde.org/D12060