Revert two commits for bug#19032

This reverts:

- commit 585fe00557, which is titled
  "Fix up previous icomplete-show-matches-on-no-input change"

- commit 1b8d369c38, which is titled
  "Change icomplete-show-matches-on-no-input behavior".

A simpler, less intrusive fix follows shortly after this commit.

lisp/icomplete.el

@@ -75,11 +75,7 @@ everything preceding the ~/ is discarded so the interactive
 selection process starts again from the user's $HOME.")
 
 (defcustom icomplete-show-matches-on-no-input nil
-  "When non-nil, show completions when first prompting for input.
-This also means that if you traverse the list of completions with
-commands like `C-.' and just hit `C-j' (enter) without typing any
-characters, the match under point will be chosen instead of the
-default."
+  "When non-nil, show completions when first prompting for input."
   :type 'boolean
   :version "24.4")
 
@@ -755,10 +751,7 @@ matches exist."
 		(push comp prospects)
 	      (setq limit t))))
 	(setq prospects (nreverse prospects))
-        ;; Return the first match if the user hits enter.
-        (when icomplete-show-matches-on-no-input
-          (setq-local completion-content-when-empty (car prospects)))
-        ;; Decorate first of the prospects.
+	;; Decorate first of the prospects.
 	(when prospects
 	  (let ((first (copy-sequence (pop prospects))))
 	    (put-text-property 0 (length first)

lisp/minibuffer.el

@@ -1119,7 +1119,6 @@ completion candidates than this number."
 (defvar-local completion-all-sorted-completions nil)
 (defvar-local completion--all-sorted-completions-location nil)
 (defvar completion-cycling nil)      ;Function that takes down the cycling map.
-(defvar completion-content-when-empty nil)
 
 (defvar completion-fail-discreetly nil
   "If non-nil, stay quiet when there  is no match.")
@@ -1504,13 +1503,8 @@ If `minibuffer-completion-confirm' is `confirm-after-completion',
 COMPLETION-FUNCTION is called if the current buffer's content does not
 appear to be a match."
     (cond
-     ;; Allow user to specify null string.  In the case that
-     ;; `completion-content-when-empty' is set, use that instead.
-     ((= beg end)
-      (when completion-content-when-empty
-        (completion--replace beg end completion-content-when-empty))
-      (funcall exit-function))
-
+     ;; Allow user to specify null string
+   ((= beg end) (funcall exit-function))
      ((test-completion (buffer-substring beg end)
                        minibuffer-completion-table
                        minibuffer-completion-predicate)