Cleanup code for finishing undo/redo of image operations

Summary:
D11715 introduced a function `finishUndoJob()` which adds a timer
delay for `QUndoStack` updates. After enabling redo in D12105 this
separate function for undo operations is not needed anymore and all
operation types can be handled in `finish()`.

FIXED-IN: 18.04.2

Test Plan:
# Use different image operations to edit one or more image(s)
# Check that undo/redo behaviour did not change

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: huoni, rkflx

Differential Revision: https://phabricator.kde.org/D12824