fix: drop s library dependency

The function `s-equals-p` from the s library was referenced in the code but the dependency had not
been declared.

Since the referenced function is nothing but a wrapper around `string-equal` (see below), this
commit refactors the code to call `string-equal` directly and drop the dependency altogether.

  (defun s-equals? (s1 s2)
    "Is S1 equal to S2?

  This is a simple wrapper around the built-in `string-equal'."
    (declare (pure t) (side-effect-free t))
    (string-equal s1 s2))

Author: midsbie

copilot.el

@@ -743,7 +743,7 @@ automatically, browse to %s." user-code verification-uri))
       (setq copilot--completion-cache result))
     (let ((completions (cl-remove-duplicates (plist-get result :completions)
                                              :key (lambda (x) (plist-get x :text))
-                                             :test #'s-equals-p)))
+                                             :test #'string-equal)))
       (cond ((seq-empty-p completions)
              (copilot--log 'warning "No completion is available."))
             ((= (length completions) 1)