Remove dynamic declaration of 'save-match-data-internal'

* lisp/subr.el: Remove defvar which has no relevance today; it can
very well be a lexical variable.

lisp/subr.el

@@ -886,10 +886,6 @@ side-effects, and the argument LIST is not modified."
 
 ;;;; Keymap support.
 
-;; Declare before first use of `save-match-data',
-;; where it is used internally.
-(defvar save-match-data-internal)
-
 (defun kbd (keys)
   "Convert KEYS to the internal Emacs key representation.
 KEYS should be a string in the format returned by commands such