Remove one of recently added warnings abound binding keys in Isearch maps

* lisp/isearch.el (minibuffer-local-isearch-map): Remove comments
which warn against wantonly rebinding unbound keys from
irrelevant keymap.
https://lists.gnu.org/archive/html/emacs-devel/2021-01/msg00259.html
1 job for emacs-27 in 37 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
canceled #10095
test-all

00:37:33