Remove the config section related to Monolog

[ste93cry]

I cannot think of any reason for keeping the sentry.monolog configuration option as it doesn't offer any benefit over registering the service by hands. Since changing the Monolog configuration is too hard and cannot be done in an automatic way, the user has anyway to edit the configuration files to add the service to the Monolog handlers stack and at that point having to write the service definition imo is the least. Last, registering the service by hands will make it possible to set environment variables as values of the constructor of the handler, rather than us having to provide workarounds to support this feature in the configuration (#384)

[Jean85]

I agree with this. It will fail hard on update so it's easy to change, and we can later reintroduce it better if we will work on #337