Remove log4j-codegen (#151)

The `log4j-codegen` module was moved from Log4j Core 2 and generates code for custom loggers.

Since:

1. We don't recommend the usage of custom logger wrappers.
2. The functionality of a custom logger wrapper can be easily replaced by the usage of the appropriate `Level` parameter.

I propose to remove the `log4j-codegen` module.

Author: ppkarwasz