Unnecessary config file creation

[neutrinoceros]

Bug report

As discussed in #3044, we are still creating a yt config file at import time when none is already available, but this is an implementation detail inherited from before we switch to toml config files and apparently is not necessary any more (according to @cphyc).
I think there is no good reason to keep this behaviour now.

Additionally, this behaviour makes working with a local configuration file pretty clunky. For instance if one doesn't have a global file (yet), but runs

echo "[yt]\nserialize = false" > yt.toml
yt config list

they get

Yt detected a local and a global configuration file, refusing to proceed.
Local config file: /Users/robcleme/dev/yt-project/yt/yt.toml
Global config file: /Users/robcleme/.config/yt/yt.toml
Specify which one you want to use using the `--local` or the `--global` flags.

[neutrinoceros]

I attempted to fix this in #3223, but it seems that solving this problem would require spending more time decoupling the following:

• yt/__init__.py
• yt/utilities/configure.py
• yt/utilities/logger.py
• yt/config.py

and probably other modules.
In short, I think the root problem is that a bunch of code is run at import time that require the globale ytcfg object to be set, forcing us to parse configuration mid initialisation. Forcing a config file to exists is part of the current solution to make this work.