DynamicMenuContributions

To fix bug regression from bug 518036 we should not call
clearModelToContribution() on removeDynamicMenuContributions(): this
will remove also not dynamic children, which results in duplicated
entries next time menu is opened.

Change-Id: I557d7b3e215d666842f0d6348c8ca0a754e319d9
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>