[HttpKernel] remove unneeded strtoupper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

It is already uppercased by the exception itself: https://github.com/symfony/symfony/blob/2.2/src/Symfony/Component/Routing/Exception/MethodNotAllowedException.php#L32