Backend Calendar for staff memebrs optimisations

[Valentina R]

On the Calendar, we should remove the 'Staff' calendar - the one where you can see all staff members for a specific day. Since staff members should only be able to see their appointments, this view should not be visible to them.

We should also remove the staff filter dropdown (since the user cannot select anything, there is no use of having it there).

Reported here: https://bookingwp.com/forums/topic/restict-shopstaff-user-role-to-access-other-staff-calendars/

[Željan Topić]

Added with 4.4.6