Drop support for $AUTO_REQUIREMENTS

[yajo]

This feature was introduced in #71, but time has proven it buggy and not much useful.

This feature has a chicken-and-egg problem in the devel environment: code doesn't exist at build time, but it is expected to be able to find the requirements.txt files inside it.

It confuses the developer and creates divergencies between devel and test/prod environments (where that problem doesn't exist).

Also it's breaking #174, which is a more important and actually used feature.

The best fix is to drop support for this. If you really want to have requirements from remotes, you can add this to your dependencies/pip.txt file:

# Assuming you are installing all from server-tools
-r https://raw.githubusercontent.com/OCA/server-tools/11.0/requirements.txt

It's a better choice because:

• It's basic official pip.
• It's explicit.
• It works fine across environments.