sane-backends: revert BUILDONLY flag

BUILDONLY was disabling SANE backends (drivers) build.

Closes #14484

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>

utils/sane-backends/Makefile

@@ -43,7 +43,6 @@ $(call Package/sane-backends/Default)
   CATEGORY:=Utilities
   TITLE+= (drivers)
   DEPENDS:=+ALL:sane-backends-all
-  BUILDONLY:=1
 endef
 
 define Package/sane-backends/description