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>

package/dialog/dialog.mk

@@ -21,9 +21,4 @@ define DIALOG_INSTALL_TARGET_CMDS
 	install -c $(@D)/dialog $(TARGET_DIR)/usr/bin/dialog
 endef
 
-define DIALOG_POST_CLEAN
-	-$(MAKE) -C $(@D) clean
-	rm -f $(TARGET_DIR)/usr/bin/dialog
-endef
-
 $(eval $(autotools-package))