Simplify gptel tool registration in README

* README.org (llm-tool-collection-register-with-gptel): Simplify so as
to simply call 'gptel-make-tool'.  The latter function has a setter
that manages the internal 'gptel--known-tools' and 'gptel-tools' lists
and correctly handles duplicates.

Author: ultronozm

README.org

@@ -148,13 +148,7 @@ There will likely be many iterations necessary to get a tool to a good state. To
   (defun llm-tool-collection-register-with-gptel (tool-spec)
     "Register a tool defined by TOOL-SPEC with gptel.
   TOOL-SPEC is a plist that can be passed to `gptel-make-tool'."
-    (let ((tool (apply #'gptel-make-tool tool-spec)))
-      (setq gptel-tools
-            (cons tool (seq-remove
-                        (lambda (existing)
-                          (string= (gptel-tool-name existing)
-                                   (gptel-tool-name tool)))
-                        gptel-tools)))))
+    (apply #'gptel-make-tool tool-spec))
 
   (add-hook 'llm-tool-collection-post-define-functions
             #'llm-tool-collection-register-with-gptel)