virt-admin: get rid of LIBVIRT_DEFAULT_ADMIN_URI env var

There is a LIBVIRT_ADMIN_DEFAULT_URI environment variable
which is honored by virAdmConnectOpen and documented
in the virt-admin man page.

LIBVIRT_DEFAULT_ADMIN_URI is undocumented and this is its
only occurrence.

(cherry picked from commit df8964308e)

tools/virt-admin.c

@@ -670,7 +670,6 @@ main(int argc, char **argv)
 {
     vshControl _ctl, *ctl = &_ctl;
     vshAdmControl virtAdminCtl;
-    const char *defaultConn;
     bool ret = true;
 
     memset(ctl, 0, sizeof(vshControl));
@@ -719,9 +718,6 @@ main(int argc, char **argv)
 
     virFileActivateDirOverride(argv[0]);
 
-    if ((defaultConn = virGetEnvBlockSUID("LIBVIRT_DEFAULT_ADMIN_URI")))
-        ctl->connname = vshStrdup(ctl, defaultConn);
-
     if (!vshInit(ctl, cmdGroups, NULL))
         exit(EXIT_FAILURE);