bug#45836: cups-service-type duplicates lp group

[Leo Prikler]

Hello Guix,

it has come to my attention due to the recent reporting of #45830 and
some conversation in IRC, that cups-service-type adds an lp group,
which is already defined in %base-groups.  Since both share the same
definition, this is not too big an issue, but it prohibits us from
using a hard error for #45770.

I can currently think of two solutions: Either remove the lp group from
cups-service-type or remove it from base-groups.  Neither sounds
particularly awesome.  Perhaps we could also delete identical
duplicates before asserting that there are none for #45770, but that
sounds like a little much effort.  Any ideas how else to solve this?

Regards,
Leo