Delete most libraries obsolete since 24.1 and 24.3

Delete most libraries obsolete since 24.1 and 24.3

Emacs 24.3 was released 10 years ago.
* lisp/obsolete/abbrevlist.el:
* lisp/obsolete/assoc.el:
* lisp/obsolete/complete.el:
* lisp/obsolete/cust-print.el:
* lisp/obsolete/erc-hecomplete.el:
* lisp/obsolete/mailpost.el:
* lisp/obsolete/mouse-sel.el:
* lisp/obsolete/old-emacs-lock.el:
* lisp/obsolete/patcomp.el:
* lisp/obsolete/pc-select.el:
* lisp/obsolete/s-region.el: Delete files.  These libraries have been
obsolete since Emacs 24.1 or 24.3.  (Bug#50999)
* etc/NEWS: Announce their deletion.

* lisp/minibuffer.el (minibuffer-confirm-exit-commands):
* lisp/textmodes/rst.el: Remove references to above obsolete
libraries.