remove dead "highest_role" method

refs CNVS-18529

found this while migrating the salesforce plugin
and digging into how roles work.  It's never called. removing it.

TEST PLAN:
 - nothing should change, this method is never used

Change-Id: Ibae6d4de5cc42e5c6dcfb2746d8241b2a71257a8
Reviewed-on: https://gerrit.instructure.com/50777
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>

app/models/user.rb

@@ -2181,10 +2181,6 @@ class User < ActiveRecord::Base
   TAB_EPORTFOLIOS = 3
   TAB_HOME = 4
 
-  def highest_role
-    roles.last
-  end
-
   def roles(root_account)
     return @roles if @roles
     @roles = ['user']