Remove redundant requires of 'custom'

Remove redundant requires of 'custom'

* lisp/emacs-lisp/eieio-custom.el:
* lisp/htmlfontify.el:
* lisp/mwheel.el:
* lisp/net/eudc-vars.el:
* lisp/net/ldap.el:
* lisp/net/tramp-ftp.el:
* lisp/net/tramp-gvfs.el:
* lisp/progmodes/cwarn.el:
* lisp/progmodes/sql.el:
* lisp/savehist.el:
* lisp/textmodes/reftex.el:
* lisp/wid-browse.el: Don't require 'custom'; it is preloaded since
version 20.1.