WL-3143 Removing test case for public access

This should never happen now since it is carried out by setting a role
for public access so it is handled by the 'inherited' case.

This makes the code tidier.
WL-3143 Clear group access when turned off

Currently if you switch to site access when you have group access there
will only be changes made if roles are added or taken away. This forces
group access to be removed in the BaseContentService regardless of the
additional roles that are checked.