Unconditionally activate the occur stuff in ledger-occur-mode

Well, we still deactivate it when regex is nil, but the function
should not look at previous value of ledger-occur-mode:

- the interactive function (ledger-occur) already do it, we don't need
  to do it there,
- caller that want to deactivate the occur stuff only have to call
  ledger-occur with a nil regex
- the old behavior make ledger-reconcile to turn off occur stuff if it
  was already turn on, when what we do want is that the occur stuff
  change to the new account.

lisp/ldg-occur.el

@@ -69,13 +69,14 @@
   "A list of currently active overlays to the ledger buffer.")
 (make-variable-buffer-local 'ledger-occur-overlay-list)
 
-
 (defun ledger-occur-mode (regex buffer)
+  "Higlight transaction that match REGEX, hiding others
+
+When REGEX is nil, unhide everything, and remove higlight"
   (progn
     (set-buffer buffer)
     (setq ledger-occur-mode 
-	  (if (or ledger-occur-mode
-		  (null regex)
+	  (if (or (null regex)
 		  (zerop (length regex)))
 	      nil
 	      (concat " Ledger-Folded: " regex)))