Remove superfluous return value from 'before' callbacks

In Rails 5.0, halting a 'before' callback by returning false is
deprecated in favour of `throw(:abort)`, and in Rails 5.1 that feature
is removed.

Conveniently, all our callbacks return true, so we don't need to do
much.

See https://guides.rubyonrails.org/v5.2/upgrading_ruby_on_rails.html#halting-callback-chains-via-throw-abort

Author: tom93

app/models/contest.rb

@@ -36,7 +36,6 @@ class Contest < ActiveRecord::Base
     end
 
     update_contest_scores if finalized_at_was && finalized_at.nil?
-    true
   end
 
   # calculate contest scores again from scratch

app/models/item.rb

@@ -12,7 +12,6 @@ class Item < ActiveRecord::Base
 
   before_create do
     self.scan_token = SecureRandom.random_number(100000000)
-    true
   end
 
   def scan_token

app/models/submission.rb

@@ -33,7 +33,6 @@ def user_problem_relation
     end
     self.evaluation = points.nil? || maximum_points.nil? ? nil : (points / maximum_points).to_f
     update_test_messages
-    true
   end
 
   before_create do

app/models/user.rb

@@ -18,7 +18,6 @@ class User < ActiveRecord::Base
 
   before_save do
     self.can_change_username = false if username_changed? # can only change username once
-    true # this line should be removed once we upgrade to Rails >= 5.1, see https://guides.rubyonrails.org/5_1_release_notes.html#active-model-removals
   end
 
   has_many :problems