Remove incorrect line from performance docs

V8 does not use a single map-transition-graph for all types, and thus
the line about ordering of properties in constructor functions is
incorrect.

Change-Id: Iaf4ac02dcfc53dd7254af1a40a3c66852a78a320
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>

doc/src/qml/performance.qdoc

@@ -445,7 +445,6 @@ criteria:
 \list
 \o Avoid using eval() if at all possible
 \o Do not delete properties of objects
-\o Try to maintain property-assignment order in different constructor functions
 \endlist
 
 \section1 Common Interface Elements