refactor: remove ui plugin code which was relocated to a new repository

[reubenmiller]

Move the Cumulocity UI plugin to an external repository: https://github.com/thin-edge/tedge-container-plugin-ui

Separating out the project will allow for a cleaner release process as the two different artifacts release cycles (especially when it comes to security alerts).

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
30	0	0	30	100	7m32.597676999s

Passed Tests

Name	⏱️ Duration	Suite
Get Container Logs	6.351 s	Container-Logs
Get Container Logs with only last N lines	0.513 s	Container-Logs
Remove Container	7.427 s	Container-Remove
Remove Container Non Existent Container Should Not Through An Error	0.056 s	Container-Remove
Update to tedge-container-plugin-ng	25.623 s	Installation
Check for Update	7.608 s	Operations-Clone
Clone Existing Container	27.681 s	Operations-Clone
Clone Existing Container by Timeout Whilst Waiting For Exit	18.052 s	Operations-Clone
Clone Existing Container but Waiting For Exit	23.741 s	Operations-Clone
Ignore Containers With Given Label	21.057 s	Operations-Clone
Install/uninstall container package from private repository - credentials file	14.621 s	Operations-Private-Registries
Install/uninstall container package from private repository - credentials script	14.871 s	Operations-Private-Registries
Install/uninstall container package from private repository - credentials script with cache	21.895 s	Operations-Private-Registries
Install/uninstall container package from private repository - engine credentials	18.878 s	Operations-Private-Registries
Install/uninstall container package from private repository - docker from docker	37.960 s	Operations-Private-Registries
Get Configuration	5.302 s	Operations
Install/uninstall container-group package	15.015 s	Operations
Install invalid container-group	2.978 s	Operations
Install container-group with multiple files - app1	12.245 s	Operations
Install container-group with multiple files - app2	12.217 s	Operations
Install/uninstall container package	17.056 s	Operations
Install/uninstall container package from file	11.811 s	Operations
Manual container creation/deletion	20.462 s	Operations
Manual container creation/deletion with error on run	7.872 s	Operations
Manual container created and then killed	10.390 s	Operations
Self Update Is Present Using Self Type	0.147 s	Self
Self Update Is Present Using Container Type	0.157 s	Self
Self Update Is Not Present	0.149 s	Self
Service status	19.034 s	Telemetry-Main
Sends measurements	71.427 s	Telemetry-Main