Align personal snippets delete ability

Summary

At the moment, in the PersonalSnippetPolicy we have several abilities: :read_personal_snippet, :update_personal_snippet, :destroy_personal_snippet, and :admin_personal_snippet.

Based on those abilities, the one used to check if the user can delete a personal snippet should be destroy_personal_snippet. Nevertheless, the only place where we use this ability is in the snippets API. Everywhere else, the controller and the views, we check for the admin_personal_snippet.

A similar ability is the one check if the user can delete project snippets, admin_project_snippet.

Possible fixes

We should remove the destroy_personal_snippet and use always admin_personal_snippet.