Skip adding default remotes if CONAN_NO_DEFAULT_REMOTES is set to True/1

[DailyMats]

Changelog: Feature
Docs: conan-io/docs#3583

If you mostly use Conan to build locally, using conanfiles you're handling (and exporting) yourself, and don't necessary set up a remote, there is no point in adding conancenter as a remote, and having Conan querying it for those.

[CLAassistant]

All committers have signed the CLA.

[memsharded]

Thanks for your contribution @DailyMats

However, I am afraid that this kind of usage of env-vars to configure Conan behavior is no longer used in Conan 2.0.

There are already some different mechanisms to avoid using the conan default defined remote:

• You can easily remove it with conan remote remove "*", or disable it with conan remote disable "*"
• The recommended way to manage remotes and other configuration items is via conan config install, that can use an emptied remotes.json file to clear remotes (and define your own ones if desired)
• Commands can use the --no-remotes argument to disable remotes

[memsharded]

I am closing this as not planned, for the reasons above.
Thanks very much for your contribution anyway!