T6203: remove obsoleted xml lib (backport #3255)

[mergify]

Change Summary

The vyos.xml modules and scripts were replaced with vyos.xml_ref to (1) address a bug with defaults under tag nodes and (2) support extensions (T5218). Remove missed remaining reference in op-mode script otp.py and remove obsoleted code.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T5218

Related PR(s)

Component(s) name

Proposed changes

How to test

git ls-files *.py | xargs pylint | grep E0611 | grep xml

Smoketest result

make test 2>&1|tee out.test
tail -n 12 out.test
DEBUG - EXITCODE:0
 INFO - Smoketest finished successfully!
 INFO - Powering off system
DEBUG - poweroff now
 INFO - Shutting down virtual machine
 INFO - Waiting for shutdown...
DEBUG - poweroff now
 INFO - Waiting for shutdown...
DEBUG - poweroff now
 INFO - VM is shut down!
 INFO - Cleaning up
 INFO - Removing disk file: testinstall-20240406-010238-53ee.img

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3255 done by [Mergify](https://mergify.com).