mcookie: remove SOURCE variable

Now that the package infrastructure doesn't attempt to download a package
that has an empty version string, there's no need to define the SOURCE
variable in the mcookie package.

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

package/x11r7/mcookie/mcookie.mk

@@ -4,8 +4,6 @@
 #
 ################################################################################
 
-# Source included in Buildroot
-MCOOKIE_SOURCE =
 MCOOKIE_LICENSE = Public Domain
 
 define MCOOKIE_EXTRACT_CMDS