[doc] QRasterPaintEngine: remove docs for QSpan

Nothing in this class uses it, so there's no reason to document the
typedef. It will just confuse both the compiler and the user when we
add QSpan (a std::span implementation).

Pick-to: 6.4 6.2
Task-number: QTBUG-108124
Change-Id: Iff0249d2e12a05fca15998a046a0b1bf607b60be
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/gui/painting/qpaintengine_raster.cpp

@@ -276,15 +276,6 @@ QRasterPaintEnginePrivate::QRasterPaintEnginePrivate() :
     \reimp
 */
 
-/*!
-    \typedef QSpan
-    \relates QRasterPaintEngine
-
-    A struct equivalent to QT_FT_Span, containing a position (x,
-    y), the span's length in pixels and its color/coverage (a value
-    ranging from 0 to 255).
-*/
-
 /*!
     \since 4.5