Fix delete-backward-char in haskell-indentation (fixes #211)

The problem in #211 is that while delete-char should only delete the
region if delete-selection-mode is enabled, delete-backward-char
deletes if the region is active and delete-active-region is enabled.
Its default value is t.

haskell-indentation.el

@@ -378,8 +378,8 @@ Preserves indentation and removes extra whitespace"
    (cond
     ((haskell-indentation-outside-bird-line)
      (delete-char (- n)))
-    ((and delete-selection-mode
-          mark-active
+    ((and (use-region-p)
+          delete-active-region
           (not (= (point) (mark))))
      (delete-region (mark) (point)))
     ((or (= (haskell-current-column) 0)