util-linux: drop BR2_PACKAGE_BUSYBOX_SHOW_OTHERS dependency

Drop the dependency as it isn't really nice that other packages need
to select it, and kconf complains about it:

warning: (BR2_PACKAGE_E2FSPROGS && BR2_PACKAGE_USBMOUNT) selects
 BR2_PACKAGE_BUSYBOX_SHOW_OTHERS which has unmet direct dependencies
 (!BR2_PACKAGE_BUSYBOX)

Instead ensure that only the libraries are built by default, so
busybox applets are still used unless explicitly configured.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>