remove not needed logger tests

The `isset` checks on `$app['logger']` make no sense since `Application` always sets the logger (to `null`). Checks for `!empty` would be more safe/complete (altough `!empty && instance_of('LoggerInterface')` would be more ideal in some cases).
Updating the unit tests is more complicated because some code under test depends on classes outside of Silex.
Hacking the `class_exists` function or use `mocking` of classes that do not exists (@see https://github.com/sebastianbergmann/phpunit-mock-objects/issues/12) would effect the whole test suit (as @stof pointed out).