package/zsh: remove busybox show others dependency

package/zsh: remove busybox show others dependency

Back in 2013, zsh was added to Buildroot [1] with the dependency on
BR2_PACKAGE_BUSYBOX_SHOW_OTHERS. This dependency was keept with the
refactoring [2].
But Busybox doesn't provide a zsh shell in the firt place.
This dependency can be removed.

[1] 3d19b7ef598f3534b46cf0746a81bc437579127c
[2] 1dbd7b991025a8bf39ab5d25c2fd97f47f084bac

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Phil Eichinger <phil.eichinger@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>