Remove duplicated content from Azure.Core Response.md

[scottaddie]

Addresses dotnet/docs#24888

Summary of changes

• Remove the content from the Markdown file that now exists in a localized and more discoverable form on docs.microsoft.com (https://docs.microsoft.com/dotnet/azure/sdk/pagination).
• Remove unit tests for the deleted snippets.
• Include a link in README.md to the official pagination doc.
• Remove 2 unused snippets.