This reverts commit 26765b8b.

A different clipboard is actually useful for the following use case:

1 open an archive in file-roller
2 select a list of files
3 context menu -> cut
4 context menu -> paste
5 the paste dialog asks the user a destination
6 the user wants to copy and paste the destination from a selected
  text from another application

the last step can't be done if a single clipboard is used, because
copying a text to the clipboard would overwrite the clipbaord content
that contains the files cut in step 3.