ifconfig.8: Deduplicate groupname documentation

-G and -g flags are already described somewhere else in the manual.

MFC after:	2 weeks

sbin/ifconfig/ifconfig.8

@@ -28,7 +28,7 @@
 .\"     From: @(#)ifconfig.8	8.3 (Berkeley) 1/5/94
 .\" $FreeBSD$
 .\"
-.Dd March 29, 2022
+.Dd March 30, 2022
 .Dt IFCONFIG 8
 .Os
 .Sh NAME
@@ -183,8 +183,6 @@ parameter is a string of the form
 .Dq name unit ,
 for example,
 .Dq Li em0 .
-.It Ar groupname
-List the interfaces in the given group.
 .El
 .Pp
 The output format of