[cross_file] Delete. (flutter#3698)

The package now lives in flutter/packages.

Author: ditman

CODEOWNERS

@@ -6,7 +6,6 @@
 
 
 packages/camera/**                             @bparrishMines
-packages/cross_file/**                         @ditman @mvanbeusekom
 packages/file_selector/**                      @ditman
 packages/google_maps_flutter/**                @cyanglaz
 packages/image_picker/**                       @cyanglaz