QKeySequence: remove a clause guarding an impossible condition

All code paths leading up to this line unconditionally dereference the pointer,
so it cannot be nullptr.

Remove the check. It only confuses readers of the code and probably will trigger
warnings with tomorrow's compilers.

Change-Id: I31a643738e951da9c04f0091645016276b047761
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qkeysequence.cpp

@@ -1100,7 +1100,6 @@ int QKeySequencePrivate::decodeString(const QString &str, QKeySequence::Sequence
                      << QModifKeyName(Qt::KeypadModifier, QLatin1String("num+"));
         }
     }
-    if (!gmodifs) return ret;
 
 
     QVector<QModifKeyName> modifs;