fix hot reload of theme when removing a package

Steps to reproduce:

• make a studio project with a custom theme (change the logo for instance)
• install the package from admin center > update center
• click on main tab "document management" => custom logo is displayed
• go back to the admin center > update center > local packages and uninstall the studio package
• click on main tab "document management" => theme is completely broken as css are trying to be loaded from the removed theme

=> ThemeService#reload does not do the job properly as it keeps old extensions that have been removed