Revert "More for Qt 5.12"

This reverts commit 3e9f33bb3e.

Differential Revision: https://phabricator.kde.org/D29147

libkwineffects/kwinglplatform.cpp

@@ -686,12 +686,8 @@ void GLPlatform::detect(OpenGLPlatformInterface platformInterface)
         }
     } else {
         const QByteArray extensions = (const char *) glGetString(GL_EXTENSIONS);
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
-        const QList<QByteArray> extensionsList = extensions.split(' ');
+        QList<QByteArray> extensionsList = extensions.split(' ');
         m_extensions = {extensionsList.constBegin(), extensionsList.constEnd()};
-#else
-        m_extensions = extensions.split(' ').toSet();
-#endif
     }
 
     // Parse the Mesa version