Remove redundant position sequence from Track factory

This sequence pre-dates the use of `acts_as_list` on `Track` and is no
longer necessary. Furthermore it was actually giving unrealistic data in
tests, because the sequence is *global* rather than per album.

test/factories/tracks.rb

@@ -4,7 +4,6 @@
   factory :track do
     album
     sequence(:title) { |n| "Never gonna give you up #{n}" }
-    sequence(:position) { |n| n }
 
     after(:build) do |track|
       track.original.attach(