[2.5][FrameworkBundle] Redirect server output to /dev/null in case no verbosity is needed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no 
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

This should reduce I/Os
The verbosity can change while the callback is called, can't it? So I'm not sure it can safely be removed.