Remove beta distinction from Users API endpoints

CLOSES: LS-1529
flag=none

Test plan:
- bundle exec rake doc: api
- View Users API
- View Get dashboard positions and
Update dashboard positions endpoints without
the beta label reference

Change-Id: I2c5fe34607f855d34f9226ca78ec1b6d1473505f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/249563
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Erin Hallmark <erin@instructure.com>
Product-Review: Erin Hallmark <erin@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>

app/controllers/users_controller.rb

@@ -1691,7 +1691,6 @@ def set_custom_color
   end
 
   # @API Get dashboard positions
-  # @beta
   #
   # Returns all dashboard positions that have been saved for a user.
   #
@@ -1716,7 +1715,6 @@ def get_dashboard_positions
   end
 
   # @API Update dashboard positions
-  # @beta
   #
   # Updates the dashboard positions for a user for a given context.  This allows
   # positions for the dashboard cards and elsewhere to be customized on a per