Remove nuget.config to address pipeline warnings (Azure#5829)

We added a nuget.config file some time back because we needed to pull from multiple nuget feeds: the regular public feed and an internal feed needed occasionally/temporarily for special drops of the IoT SDK. When Microsoft began scanning for config files that might be subject to a substitution attack, we took a few different steps over time to ensure compliance.

Now we're beginning to see warnings in our pipeline because our config file points to the public feed, _even though its the only configured feed_. I suspect the scans are geared towards internal projects using OSS, rather than for public-facing projects which are themselves OSS.

Since we don't need any special feeds, I believe we can avoid the pipeline warnings by removing the config file altogether.