Revert "QDistanceField: add missing copy assignment operator"

The reverts commit 0624c99ea3, which
fixed a GCC 9 -Wdeprecated-copy warning by adding the copy assignment
operator.

The 5.12 change 0e162315 fixed the same warning by removing the copy
ctor. The merge ef37ab99 removed the copy ctor, but kept the
assignment operator added by 0624c99e, bringing the original warning
back, but with the roles of copy ctor and assignment operator
reversed.

One has to give.

Change-Id: Ib32841df94a5ff80402a68b5fe776eb82e94136f
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/gui/text/qdistancefield.cpp

@@ -899,9 +899,6 @@ QDistanceField::QDistanceField(int width, int height)
 {
 }
 
-QDistanceField &QDistanceField::operator=(const QDistanceField &)
-    = default;
-
 QDistanceField::QDistanceField(const QRawFont &font, glyph_t glyph, bool doubleResolution)
 {
     setGlyph(font, glyph, doubleResolution);

src/gui/text/qdistancefield_p.h

@@ -94,7 +94,6 @@ public:
     QDistanceField(const QRawFont &font, glyph_t glyph, bool doubleResolution = false);
     QDistanceField(QFontEngine *fontEngine, glyph_t glyph, bool doubleResolution = false);
     QDistanceField(const QPainterPath &path, glyph_t glyph, bool doubleResolution = false);
-    QDistanceField &operator=(const QDistanceField &other);
 
     bool isNull() const;