Disable filtering of commands in M-x completion

Disable filtering of commands in M-x completion

This makes the default behavior like it was before:
M-x completion doesn't filter out any commands.  To
have commands filtered based on their relevance to the
current buffer's modes, customize the option
'read-extended-command-predicate' to call
'command-completion-default-include-p'.
* doc/lispref/commands.texi (Interactive Call):
* doc/emacs/m-x.texi (M-x): Update the description of
'read-extended-command-predicate' and improve wording.

* etc/NEWS: Update the entry about
'read-extended-command-predicate'.

* lisp/simple.el (read-extended-command-predicate): Change default
value to nil.  Update doc string.  Add :group.
(read-extended-command): Handle nil as meaning to apply
no-filtering.