dialog: remove useless POST_CLEAN command

For some reason, the dialog package has a DIALOG_POST_CLEAN variable,
which doesn't match any of the variables understood by the
infrastructure. This commit gets rid of it.

This was noticed while reviewing the packaging of python-dialog.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>