Shared folder accessible after removed from list of shared folders

[A_Dombeck]

When removing a 'permanent' shared folder while it is mounted, it is removed from the list of shared folders but it's still fully accessible in the VM. There is a pop up warning that this will result in 'broken shared folder', which only offers to click 'OK'. The folder is even accessible after rebooting the guest system and host system. Tested this with Linux and Windows guest systems.

Steps to reproduce:

1. Add shared folder in shared folder settings, check 'Make Permanent'
2. Mount shared folder in guest system
3. Remove shared folder in shared folder settings