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 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