generic: don't explicitly enable busybox sha256sum applet

LEDE has sha256sum enabled by default.

targets/generic

@@ -3,7 +3,6 @@ config '# CONFIG_TARGET_ROOTFS_INITRAMFS is not set'
 config 'CONFIG_ALL_NONSHARED=y'
 
 config 'CONFIG_BUSYBOX_CUSTOM=y'
-config 'CONFIG_BUSYBOX_CONFIG_SHA256SUM=y'
 config '# CONFIG_BUSYBOX_CONFIG_FEATURE_PREFER_IPV4_ADDRESS is not set'
 
 config '# CONFIG_KMOD_BATMAN_ADV_MCAST is not set'