remove example of external dtn IP

Use of dtn.rc.colorado.edu requires a per IP address firewall exception, we are removing mention of the external dtn IP and advising users to just use dtn.rc.int.colorado.edu from now on.

docs/compute/data-transfer.md

@@ -113,15 +113,13 @@ would like to send the file to.
 
 scp <path-to-file> <username>@login.rc.colorado.edu:<target-path>    # using a login node
 scp <path-to-file> <username>@dtn.rc.int.colorado.edu:<target-path>  # using DTN while on campus network
-scp <path-to-file> <username>@dtn.rc.colorado.edu:<target-path>      # using DTN while outside campus network
 ```
 
 ```bash
 # Copying files from Research Computing to a local workstation
 
 scp <username>@login.rc.colorado.edu:<path-to-file> <target-path>    # using a login node
 scp <username>@dtn.rc.int.colorado.edu:<path-to-file> <target-path>  # using DTN while on campus network
-scp <username>@dtn.rc.colorado.edu:<path-to-file> <target-path>      # using DTN while outside campus network
 ``` 
 
 Windows users can access scp through PowerShell or using a GUI
@@ -152,15 +150,13 @@ would like to send the file to.
 
 rsync -r <path-to-directory> <username>@login.rc.colorado.edu:<target-path>    # using a login node
 rsync -r <path-to-directory> <username>@dtn.rc.int.colorado.edu:<target-path>  # using DTN while on campus network
-rsync -r <path-to-directory> <username>@dtn.rc.colorado.edu:<target-path>      # using DTN while outside campus network
 ```
 
 ```bash
 # Synchronizing from Research Computing to a local workstation
 
 rsync -r <username>@login.rc.colorado.edu:<path-to-directory> <target-path>    # using a login node
 rsync -r <username>@dtn.rc.int.colorado.edu:<path-to-directory> <target-path>  # using DTN while on campus network
-rsync -r <username>@dtn.rc.colorado.edu:<path-to-directory> <target-path>      # using DTN while outside campus network
 ```
 
 rsync is not available on Windows by default, but [may be installed
@@ -183,7 +179,6 @@ between a local system and Research Computing resources.
 ```bash
 sftp <username>@login.rc.colorado.edu    # using a login node
 sftp <username>@dtn.rc.int.colorado.edu  # using DTN while on campus network
-sftp <username>@dtn.rc.colorado.edu      # using DTN while outside campus network
 ```
 
 We can then use various commands to traverse and manipulate both local