defconfig: remove busybox config setting

Remove BR2_PACKAGE_BUSYBOX_CONFIG so the default config for the
selected version gets used. Got broken by r21257.

.defconfig

@@ -258,7 +258,6 @@ BR2_BUSYBOX_VERSION_1_9_X=y
 # BR2_PACKAGE_BUSYBOX_SNAPSHOT is not set
 BR2_BUSYBOX_VERSION="1.9.0"
 BR2_PACKAGE_BUSYBOX_INSTALL_SYMLINKS=y
-BR2_PACKAGE_BUSYBOX_CONFIG="package/busybox/busybox-1.9.0.config"
 BR2_PACKAGE_BUSYBOX_HIDE_OTHERS=y
 # BR2_PACKAGE_BUSYBOX_SKELETON is not set