Remove declaration of QRasterPlatformPixmap::fromFile()

Remove declaration of QRasterPlatformPixmap::fromFile()

This function is not implemented, and the declaration hides the virtual
QPlatformPixtmap::fromFile():

../../include/QtGui/5.0.0/QtGui/private/../../../../../src/gui/image/qpixmap_raster_p.h:70:10:
warning:
      'QRasterPlatformPixmap::fromFile' hides overloaded virtual function
      [-Woverloaded-virtual]
    void fromFile(const QString &filename, Qt::ImageConversionFlags flags);
         ^
../../include/QtGui/../../src/gui/image/qplatformpixmap_qpa.h:90:18:
note:
      hidden overloaded virtual function 'QPlatformPixmap::fromFile' declared
      here
    virtual bool fromFile(const QString &filename, const char *format,
                 ^

Change-Id: Iedbc4acd9f9218f8fe72a44a9eff6a35b5494d75
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>