qgscompoundcurve: Remove uneeded clearCache call in operator=

Indeed `QgsCurve::operator=()` is already called. As,
`QgsCurve` does not override `operator=`,
`QgsAbstractGeometry::operator=()` is called. This operator calls the
`clear()` method which already clears the cache and the existing
curves.

(cherry picked from commit 04b1dd7)

src/core/geometry/qgscompoundcurve.cpp

@@ -119,11 +119,11 @@ QgsCompoundCurve::QgsCompoundCurve( const QgsCompoundCurve &curve ): QgsCurve( c
   }
 }
 
+// cppcheck-suppress operatorEqVarError
 QgsCompoundCurve &QgsCompoundCurve::operator=( const QgsCompoundCurve &curve )
 {
   if ( &curve != this )
   {
-    clearCache();
     QgsCurve::operator=( curve );
     for ( const QgsCurve *c : curve.mCurves )
     {

tests/src/core/geometry/testqgscompoundcurve.cpp

@@ -619,6 +619,13 @@ void TestQgsCompoundCurve::assignment()
 
   QVERIFY( cc1 != cc2 );
 
+  QgsLineString ls2;
+  ls2.setPoints( QgsPointSequence() << QgsPoint( Qgis::WkbType::PointM, 3, 4, 5, 6 )
+                 << QgsPoint( Qgis::WkbType::PointM, 1 / 2.0, 5 / 7.0, 1, 3 )
+                 << QgsPoint( Qgis::WkbType::PointM, 2, 3, 5, 7 ) );
+  cc2.addCurve( ls2.clone() );
+  QVERIFY( cc1 != cc2 );
+
   cc2 = cc1;
   QCOMPARE( cc1, cc2 );
 }