Fix assert error caused by MapItemView.

Use deleteLater() instead of delete to avoid potential assert error
(ASSERT: "context() && engine()" in file qml/qqmlboundsignal.cpp)
when a map item is deleted.

Change-Id: I86252077a3b0a0ce4b90e194c2e2c3bbd1214ef0
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>