Update developer docs to no longer support VCPKG_ROOT as a mechanism to set/find global versions of the vcpkg repo

[ahsonkhan]

https://github.com/Azure/azure-sdk-for-c/blob/cd62e2b78a396c3d5478d3b60c046adb72687274/README.md#customize-the-vcpkg-dependency-integration

Other doc instances:
https://github.com/Azure/azure-sdk-for-c/search?q=VCPKG_ROOT

I am not sure what the purpose of this is, within IoT samples, but will likely need to be updated:

azure-sdk-for-c/sdk/samples/iot/templates/iothub/test-resources-post.ps1

Line 161 in 1d57678

Write-Host "##vso[task.setvariable variable=VCPKG_ROOT]:$vcpkgRoot"

cc @vhvb1989, @BillyONeal, @danieljurek, @danewalton

[ahsonkhan]

Related issues:
actions/runner-images#6376
microsoft/vcpkg-tool@0cba610