Revert "clatd.c: remove unsetenv("ANDROID_DNS_MODE")"

This reverts commit cc94e5f25303bc0ffdc2e327b56564f59f48804f.

Reason for revert: XLAT doesn't work in permissive mode

Bug: 69863844

Change-Id: I62e390b960465048fbbd68e60d0a2c9c7d2c82d3
diff --git a/clatd.c b/clatd.c
index 0b69238..c579df0 100644
--- a/clatd.c
+++ b/clatd.c
@@ -503,6 +503,11 @@
     exit(1);
   }
 
+  // When run from netd, the environment variable ANDROID_DNS_MODE is set to
+  // "local", but that only works for the netd process itself. Removing the
+  // following line causes XLAT failure in permissive mode.
+  unsetenv("ANDROID_DNS_MODE");
+
   configure_interface(uplink_interface, plat_prefix, &tunnel, net_id);
 
   update_clat_ipv6_address(&tunnel, uplink_interface);