maint: remove maintainer out of date files

* BUGS: very much out of date.  Important notes should go on the
  Changes files.  For not fixed issues we have a public bug tracker.
* DEPENDENCIES: not only it is out of date, this file can be
  generated automatically with maintenance/dependencies.pl
* DEPRECATED: this all happened many years ago.  Also, such notices
  should go in the Changes file.
* maintenance/big_split/*: kinda pointless, no one ever did this.
  A lot of this modules are no longer here.