Panel: Avoid setting solid background when compositing is disabled
requested to merge ngraham/plasma-desktop:backport-20cb61789fad5e9ac5f155a841d2c686a80cf875 into Plasma/5.26
The svg with "solid/" style would override "opaque/" which is preferred when compositing is disabled. Since we don't have transparent panels when compositing is disabled, we don't need adaptive transparency at all.
BUG: 451451 FIXED-IN: 5.26.4 (cherry picked from commit 20cb6178)