Remove nameserver from resolv.conf at the end of AMI build

The conditional statement avoids cleaning if the `/etc/resolv.conf` is a symbolic link. It is a symbolic link when it is managed by other systems.

Cleaning the `/etc/resolv.conf` speeds up instance launch because it wouldn't try to use name server from the AMI creation environment. The delay was shown in `/var/log/cloud-init.log`:
```
2025-03-19 16:00:07,721 - util.py[DEBUG]: Resolving URL: http://169.254.169.254 took 40.099 seconds
2025-03-19 16:00:07,721 - util.py[DEBUG]: Resolving URL: http://[fd00:ec2::254] took 0.000 seconds
2025-03-19 16:00:17,731 - util.py[DEBUG]: Resolving URL: http://instance-data.:8773 took 10.010 seconds
```

Example content of `/etc/resolv.conf`:
```
cat /etc/resolv.conf
# Generated by NetworkManager
search ec2.internal
nameserver 192.168.0.2
```

Signed-off-by: Hanwen <[email protected]>

Author: hanwen-cluster

cookbooks/aws-parallelcluster-platform/files/ami_cleanup.sh

@@ -22,7 +22,7 @@ fi
 
 # Clean resolv.conf if it's not managed by system
 if [ ! -L "/etc/resolv.conf" ]; then
-    echo -n > /etc/resolv.conf
+    sed -i '/^nameserver/d' /etc/resolv.conf
 fi
 
 find /var/log -type f -exec /bin/rm -v {} \;