Remove support for some experimental languages in 2.0

Our support for Python and PHP is too limited to be used beyond a demo. Even though we do mark it as experimental, we've found that users still try to build on them, leading to a poor experience (e.g. https://github.com/Azure/Azure-Functions/issues/419).

We can't change that in 1.0, but we do have the option of removing them in 2.0. We can then bring them back later when they are built on a proper model.

Let's discuss.

Start by reading the discussion on issue #1744 and reviewing how Python and PHP support is represented in the Azure Functions host. The issue does not name files, tests, or an implementation scope; completion would require an agreed decision and a defined removal plan before code changes can begin.

Written by the indexing model from the issue text.