replacements: don't delete replaced files.

Don't delete replaced files after 30 days. There are only about 30k
replacements in total, so the cost of keeping replaced files is
negligible. It was also wrong because the media asset wasn't destroyed
too, so there were active media assets with missing files.

test/functional/post_replacements_controller_test.rb

@@ -25,9 +25,6 @@ class PostReplacementsControllerTest < ActionDispatch::IntegrationTest
           assert_response :success
         end
 
-        travel(PostReplacement::DELETION_GRACE_PERIOD + 1.day)
-        perform_enqueued_jobs
-
         assert_equal("https://cdn.donmai.us/original/d3/4e/d34e4cf0a437a5d65f8e82b7bcd02606.jpg", @post.reload.source)
         assert_equal("d34e4cf0a437a5d65f8e82b7bcd02606", @post.md5)
         assert_equal("d34e4cf0a437a5d65f8e82b7bcd02606", Digest::MD5.file(@post.file(:original)).hexdigest)