Remove softkey API, it was only implemented for Symbian.

Remove softkey API, it was only implemented for Symbian.

It is only removed from the documentation for now (I think this should
go in the beta 2).
That should liberate us to be able to change the API if needed for
other platforms that might need a soft keys API.

Once this goes in, the plan is to do the actual cleanup.

Change-Id: I9a7a3eb45597cd013fe3c4bd479ad08a25ef0b9b
Reviewed-by: Janne Anttila <janne.anttila@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>