FIX / Clean plugin configuration on uninstall function

[Mary-Clb]

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• The plugin uninstallation function was missing the deletion of glpi_configs entries belonging to the plugin.

[stonebuzz]

The CI run this SQL query

SELECT COUNT(*) FROM glpi_configs WHERE context = 'plugin:{$plugin_key}'

so

Suggested change

	'context' => 'plugin_fields',
	'context' => 'plugin:fields',

[Mary-Clb]

@stonebuzz @Rom1-B can I fix rector error in the CI in this PR ? Or should I open another one

[stonebuzz]

@Mary-Clb
Different problems require different pull requests =/