Revert "QIconLoader: use system fallback theme as the first fallback option"

This reverts commit 4710fb3528.

The theme it responsible for defining a set of good fallbacks,
and we should respect that order. We should also respect any
override the application developer does of the fallback theme,
and not unconditionally fall back to the system fallback theme.

Change-Id: I77f1a74c71a2a7db70464666b1a96a00fb4a1d8f
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/gui/image/qiconloader.cpp

@@ -384,10 +384,6 @@ QIconTheme::QIconTheme(const QString &themeName)
                 m_parents.append(fallback);
         }
 
-        // Use system fallback theme as the first fallback option
-        if (const QString &systemFallback = systemFallbackThemeName(); !systemFallback.isEmpty())
-            m_parents.prepend(systemFallback);
-
         // Ensure that all themes fall back to hicolor
         if (!m_parents.contains("hicolor"_L1))
             m_parents.append("hicolor"_L1);