Drop the "max previewable file size" limit

which was originally introduced to work around timing issues causing
us to try (and fail) to generate previews on sqlite read replicas.

test/lib/rails_ext/active_storage_blob_previewable_test.rb

@@ -1,13 +0,0 @@
-require "test_helper"
-
-class ActiveStorageBlobPreviewableTest < ActiveSupport::TestCase
-  test "small files are previewable" do
-    blob = ActiveStorage::Blob.new(filename: "test.mp4", content_type: "video/mp4", byte_size: 1024 * 1024, key: "test", checksum: "abc")
-    assert blob.previewable?
-  end
-
-  test "large files are not previewable" do
-    blob = ActiveStorage::Blob.new(filename: "test.mp4", content_type: "video/mp4", byte_size: ActiveStorageBlobPreviewable::MAX_PREVIEWABLE_SIZE + 1, key: "test", checksum: "abc")
-    assert_not blob.previewable?
-  end
-end