Revert "[9.x] Adds collection key first/last. (#41450)" (#41458)

This reverts commit 3924fbaeebbe0d3aeb587ac6fa0bf871530f0e4a.

Collection.php

@@ -389,16 +389,6 @@ public function first(callable $callback = null, $default = null)
         return Arr::first($this->items, $callback, $default);
     }
 
-    /**
-     * Get the first key of the collection.
-     *
-     * @return mixed
-     */
-    public function firstKey()
-    {
-        return array_key_first($this->items);
-    }
-
     /**
      * Get a flattened array of the items in the collection.
      *
@@ -704,16 +694,6 @@ public function last(callable $callback = null, $default = null)
         return Arr::last($this->items, $callback, $default);
     }
 
-    /**
-     * Get the last key of the collection.
-     *
-     * @return mixed
-     */
-    public function lastKey()
-    {
-        return array_key_last($this->items);
-    }
-
     /**
      * Get the values of a given key.
      *