QPaintDevice: inline keyword only on a method's declaration in-class

Putting `inline` on the definition and not the declaration could cause
some issues with some compilers, e.g. MinGW[1].

Putting `inline` on a method's declaration and definition is redundant;
I am not sure if that could cause an issue with MinGW, but just in-class
on the declaration is enough/more-idiomatic.

Amends a5953d2.

Found in API review.

[1] https://wiki.qt.io/Things_To_Look_Out_For_In_Reviews#Methods item
1.2

Pick-to: 6.8
Change-Id: Ieaba422670261330b5558f60dce2fcca2cb2723b
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/painting/qpaintdevice.cpp

@@ -21,7 +21,7 @@ QPaintDevice::~QPaintDevice()
     \internal
 */
 // ### Qt 7: Replace this workaround mechanism: virtual devicePixelRatio() and virtual metricF()
-inline double QPaintDevice::getDecodedMetricF(PaintDeviceMetric metricA, PaintDeviceMetric metricB) const
+double QPaintDevice::getDecodedMetricF(PaintDeviceMetric metricA, PaintDeviceMetric metricB) const
 {
     qint32 buf[2];
     // The Encoded metric enum values come in pairs of one odd and one even value.

src/gui/painting/qpaintdevice.h

@@ -61,7 +61,7 @@ class Q_GUI_EXPORT QPaintDevice                                // device for QPa
     virtual void initPainter(QPainter *painter) const;
     virtual QPaintDevice *redirected(QPoint *offset) const;
     virtual QPainter *sharedPainter() const;
-    double getDecodedMetricF(PaintDeviceMetric metricA, PaintDeviceMetric metricB) const;
+    inline double getDecodedMetricF(PaintDeviceMetric metricA, PaintDeviceMetric metricB) const;
 
     ushort        painters;                        // refcount
 private:
@@ -84,7 +84,7 @@ inline int QPaintDevice::devType() const
 inline bool QPaintDevice::paintingActive() const
 { return painters != 0; }
 
-inline int QPaintDevice::encodeMetricF(PaintDeviceMetric metric, double value)
+int QPaintDevice::encodeMetricF(PaintDeviceMetric metric, double value)
 {
     qint32 buf[2];
     Q_STATIC_ASSERT(sizeof(buf) == sizeof(double));