logging: drop fileConfig initialization in make_app - backout 0d4dd9380a45

0d4dd9380a45 was a bit harmful, as it might overwrite existing good logging
configuration.

0d4dd9380a45 no longer seems relevant: Testing shows that logging for `gearbox
serve` *is* activated anyway. gearbox/commands/serve.py will invoke
"setup_logging" right before "loadapp".

We must and can assume that logging has been initialized before make_app.

Reported and based on analysis by Wolfgang Scherer.