Fix uninstall

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• 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.
• This change requires a documentation update.

Description

When GLPI migrates data into the core, the itemtype column of the glpi_plugin_genericobject_types table is also updated.

For example:
PluginGenericObjectAntenne → Glpi\CustomAsset\AntenneAsset

The plugin’s uninstallation process relies on this column to locate and remove the corresponding files, classes, and MySQL tables.
Under these conditions, the plugin ends up deleting the glpi_assets_assets table.

This pull request also removes the /files/_plugins/genericobject/ directory at the end of the uninstall process, as it contains folders that were added after the uninstall method was originally implemented.

Similarly, when dropping tables, a length check is now performed on the table name to prevent the following MySQL error:

Uncaught PHP Exception RuntimeException:
"MySQL query error: Identifier name 'glpi_plugin_genericobject_xxxxxxxxxmodels' is too long (1059)"

Screenshots (if appropriate):