Revert "[9.x] Add assertUnsupportedMediaType method for assertion Unsupported Media Type (#46418)" (#46420)

This reverts commit 46199b6.

Author: taylorotwell

src/Illuminate/Testing/Concerns/AssertsStatusCodes.php

@@ -143,16 +143,6 @@ public function assertConflict()
         return $this->assertStatus(409);
     }
 
-    /**
-     * Assert that the response has a 415 "Unsupported Media Type" status code.
-     *
-     * @return $this
-     */
-    public function assertUnsupportedMediaType()
-    {
-        return $this->assertStatus(415);
-    }
-
     /**
      * Assert that the response has a 422 "Unprocessable Entity" status code.
      *