bug#6459: 24.0.50; completions-format should not have a `nil' choice

[Drew Adams]

This is the definition:
 
(defcustom completions-format nil
  "Define the appearance and sorting of completions.
If the value is `vertical', display completions sorted vertically
in columns in the *Completions* buffer.
If the value is `horizontal' or nil, display completions sorted
horizontally in alphabetical order, rather than down the screen."
  :type '(choice (const nil) (const horizontal) (const vertical))
  :group 'minibuffer
  :version "23.2")
 
There is no reason for the `nil' choice.  Please remove it and make the
default value `horizontal'.  Occam's razor (KISS) and clarity for users
argue for this.

In GNU Emacs 24.0.50.1 (i386-mingw-nt5.1.2600)
 of 2010-06-14 on 3249CTO
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4) --no-opt --cflags -Ic:/xpm/include'