ethtool: T6729: drop text based feature parsing in favour of JSON

[c-po]

Change Summary

Recent ethtool 6.10 supports JSON output for the base driver features. Remove our old text based processing code and use the machine readable output of ethtool.

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/T6729

Related PR(s)

• ethtool: T6729: upgrade to 6.10 to make use of more --json options vyos-build#769

Component(s) name

vyos.ethtool

How to test

from vyos.ethtool import Ethtool
eth0 = Ethtool('eth0')
eth0.check_speed_duplex('1000', 'full')
eth0.check_speed_duplex('auto', 'auto')

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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