package/swupdate: drop dangling SWUPDATE_BUILD_CONFIG

"6c63e4de4a boot/swupdate: don't specify .config to munge" dropped all
usages of this define but the define itself was left dangling.

Drop it now.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit ca5cd8d67554c568daee4da1f058bcf3b35b2976)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/swupdate/swupdate.mk

@@ -221,8 +221,6 @@ define SWUPDATE_SET_WEBSERVER
 endef
 endif
 
-SWUPDATE_BUILD_CONFIG = $(@D)/.config
-
 SWUPDATE_KCONFIG_FILE = $(call qstrip,$(BR2_PACKAGE_SWUPDATE_CONFIG))
 SWUPDATE_KCONFIG_EDITORS = menuconfig xconfig gconfig nconfig