Optimise QgsFeature::setAttributes

Remove the equality check "shortcut". Profiling shows that the cost
of checking the equality of the attributes well exceeds the cost
of cost always detaching the feature. And in the vast majority of
cases the equality operator would return false anyway, resulting
in a lot of wasted effort.

src/core/qgsfeature.cpp

@@ -159,9 +159,6 @@ int QgsFeature::attributeCount() const
 
 void QgsFeature::setAttributes( const QgsAttributes &attrs )
 {
-  if ( attrs == d->attributes )
-    return;
-
   d.detach();
   d->attributes = attrs;
   d->valid = true;