[weather applet] Fix unwanted ruling of implicit icon sizes on displayed size

Summary:
BUG: 398960
FIXED-IN: 5.14.0

Test Plan:
Switching icon sets between newaita, Breeze, or Oxygen no longer results in
different (too large) icon sizes to be used.

Reviewers: broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15744