Revert "[FEATURE][expressions] implicit feature->geom conversion"

This reverts commit 7cab60b.

The conversion was unreliable, as it was not correctly determining
that the feature's geometry was required and consequently not
fetching it from the provider. This isn't trivial to fix, so
I'm reverting this feature before release.

Author: nyalldawson

src/core/qgsexpression.cpp

@@ -284,15 +284,6 @@ static QgsGeometry getGeometry( const QVariant& value, QgsExpression* parent )
 {
   if ( value.canConvert<QgsGeometry>() )
     return value.value<QgsGeometry>();
-  else if ( value.canConvert<QgsFeature>() )
-  {
-    //try to grab geometry from feature
-    QgsFeature f = value.value<QgsFeature>();
-    if ( f.constGeometry() )
-    {
-      return *f.constGeometry();
-    }
-  }
 
   parent->setEvalErrorString( "Cannot convert to QgsGeometry" );
   return QgsGeometry();
@@ -1162,7 +1153,7 @@ static QVariant fcnWordwrap( const QVariantList& values, const QgsExpressionCont
 static QVariant fcnLength( const QVariantList& values, const QgsExpressionContext*, QgsExpression* parent )
 {
   // two variants, one for geometry, one for string
-  if ( values.at( 0 ).canConvert<QgsGeometry>() || values.at( 0 ).canConvert<QgsFeature>() )
+  if ( values.at( 0 ).canConvert<QgsGeometry>() )
   {
     //geometry variant
     QgsGeometry geom = getGeometry( values.at( 0 ), parent );

tests/src/core/testqgsexpression.cpp

@@ -1566,31 +1566,6 @@ class TestQgsExpression: public QObject
       QCOMPARE( out.toDouble(), result );
     }
 
-    void implicit_feature_to_geometry()
-    {
-      // test implicit feature to geometry cast
-      QgsPolyline polyline;
-      polyline << QgsPoint( 0, 0 ) << QgsPoint( 10, 0 );
-      QgsFeature feat;
-      feat.setGeometry( QgsGeometry::fromPolyline( polyline ) );
-
-      QgsExpressionContext context;
-      context.setFeature( feat );
-      QgsExpression exp1( "length($currentfeature)" );
-      QVariant result = exp1.evaluate( &context );
-      QCOMPARE( result.toDouble(), 10. );
-
-      QgsExpression exp2( "x(end_point($currentfeature))" );
-      result = exp2.evaluate( &context );
-      QCOMPARE( result.toDouble(), 10. );
-
-      // feature without geometry
-      QgsFeature feat2;
-      context.setFeature( feat2 );
-      result = exp2.evaluate( &context );
-      QVERIFY( !result.isValid() );
-    }
-
     void eval_geometry_calc()
     {
       QgsPolyline polyline, polygon_ring;