Remove branch rename notice

[jspaith]

At the end of 2021 we renamed our branch from master to main and put notes at the top of the primary readme.md with migration notes.

Since it's been +1 year, we can safely remove this notice. I am leaving the more detailed docs around just in case and since it's safely in a separate directory.

I'll close out #2172 once this goes in as well.

[ericwolz]

should we close this issue as part of the PR??

https://github.com/Azure/azure-iot-sdk-c/issues/2172

[jspaith]

@ericwol-msft - yup planning on closing that issue.