Add function to remove extra data from the registry.

* gnus-registry.el (gnus-registry-remove-extra-data): New function.

* gnus.texi (lines): Correct description of
`gnus-registry-track-extra's default value.  Mention
`gnus-registry-remove-extra-data'.
* nnml.el (nnml-request-compact-group): Don't bug out if we can't delete files

 (bug#13481).