Remove config yaml

[joelN123]

The config.yaml file contains the same information as the config.py file. So, it is possible to remove the code related to config.yaml. This is stated in https://github.com/blue-oil/blueoil/issues/1224. A second point is that loading the config.yaml can cause error https://github.com/blue-oil/blueoil/pull/1110. So this PR would also resolve that problem.

After modifying the code, I tried training, exporting, and converting. All seemed to run fine. No config.yaml appears.

p.s. in the save_yaml function, I just remove the config.yaml part, not the meta.yaml part.

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[joelN123]

@iizukak how do you think about these changes? One possible contentious point is that the save_yaml function remains, for saving the meta.yaml. I think that meta file may be important for running certain kinds of demo https://github.com/blue-oil/blueoil/blob/master/docs/usage/export.md so it's better to keep it.