Remove openafs-fileserver doc symlink in preinst

Remove openafs-fileserver doc symlink in preinst

* Remove the symlink from the openafs-fileserver doc directory to the
  openafs-client doc directory.  We used to install this symlink and
  share doc directories, but this stopped in 1.4.12+dfsg-1.  However,
  dpkg doesn't remove symlinks to a directory, so systems that had
  upgraded from the older package were overwriting openafs-client doc
  files with the openafs-fileserver versions.  Thanks, Andreas
  Beckmann.  (Closes: #694063)