Remove the code of the Azure Function

GitGitGadget consists of multiple parts:

- the actual Typescript code implementing the functionality

- a couple of Azure Pipelines that run said code

- a GitHub App, implemented in the form of an Azure Function that acts
  on the webhook events (usually PR comments containing slash commands)
  by triggering the Azure Pipelines

For various reasons, the GitHub App (i.e. the Azure Function) code had
to be extracted into its own repository over at
https://github.com/gitgitgadget/gitgitgadget-github-app.

With this commit, we remove the now-obsolete code in GitGitGadget's main
repository.

Signed-off-by: Johannes Schindelin <[email protected]>