fix(lib): revert add-transient-hook! refactor

a4b5831 removed this let-binding because I expected the function to
capture it, which seems to be the case in general, except for #6865.
Since add-transient-hook! could conceivably be used in an elisp file
with lexical-binding off, I'll take fewer chances and live with a little
redundancy.

Amend: a4b58311da
Fix: #6865

lisp/doom-lib.el

@@ -749,8 +749,9 @@ advised)."
        (defun ,fn (&rest _)
          ,(format "Transient hook for %S" (doom-unquote hook-or-function))
          ,@forms
+         (let ((sym ,hook-or-function))
            (cond ((functionp sym) (advice-remove sym #',fn))
-               ((symbolp sym)   (remove-hook sym #',fn)))
+                 ((symbolp sym)   (remove-hook sym #',fn))))
          (unintern ',fn nil))
        (cond ((functionp sym)
               (advice-add ,hook-or-function ,(if append? :after :before) #',fn))