Polish wording in crop ratio combobox

Summary:
576cfa68 enabled the clear button in the {nav Crop} ratio combobox.
This resulted in an even wider toolbar at the bottom, because the
combobox automatically adjusts its width to its widest entry,
potentially cutting off the toolbar at the right even more than before.

As all other entries also lack the "size" moniker, we can just drop it
for {nav ISO size (A4, A3, …)}. Even the original entry before the
change in a028ea5c lacked "size" in the naming.

Test Plan
Open {nav Crop} tool in {nav Advanced} mode and inspect the ratio
combobox (now with consistent wording) as well as the width of the crop
toolbar (now less wide).

Reviewers: #gwenview, ngraham, huoni

Reviewed By: ngraham

Differential Revision: https://phabricator.kde.org/D11579