remove unused Course#import_source attr_accessor

that spec code was probably meant to be a single =
but nobody ever looks at the attribute anyway

test plan: specs
flag-none
refs FOO-4198

Change-Id: If64e80e38784f837cd9ac9549cb84b268172d56f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/339359
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: August Thornton <august@instructure.com>
QA-Review: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/models/course.rb

@@ -224,7 +224,6 @@ class Course < ActiveRecord::Base
   has_many :appointment_group_contexts, as: :context, inverse_of: :context
   has_many :appointment_groups, through: :appointment_group_contexts
   has_many :appointment_participants, -> { where("workflow_state = 'locked' AND parent_calendar_event_id IS NOT NULL") }, class_name: "CalendarEvent", foreign_key: :effective_context_code, primary_key: :asset_string
-  attr_accessor :import_source
 
   has_many :content_participation_counts, as: :context, inverse_of: :context, dependent: :destroy
   has_many :poll_sessions, class_name: "Polling::PollSession", dependent: :destroy

spec/import_helper.rb

@@ -61,11 +61,8 @@ def import_example_questions
   Importers::AssessmentQuestionImporter.process_migration(hash, @migration)
 end
 
-def get_import_context(system = nil)
-  context = course_model
-  context.import_source == :webct if system == "vista"
-
-  context
+def get_import_context(_system = nil)
+  course_model
 end
 
 class ImportHelper