Remove `PIP_SUPPORT` Flag from Codebase

[srilman]

Checklist

• [X] I added a descriptive title
• [X] I searched open requests and couldn't find a duplicate

What is the idea?

In order to get rid of the Poetry dependency when specifying pip packages, we created a vendored copy of the poetry repo inside of conda-lock. However, we still have this concept of enabling and disabling PIP_SUPPORT in the codebase itself. For example, when importing the conda_lock.pypi_solver module, we do the following:

try:
    from conda_lock.pypi_solver import solve_pypi

    PIP_SUPPORT = True
except ImportError:
    PIP_SUPPORT = False

However, it is unlikely for this import to fail since the pypi_solver module now only imports required dependencies.

Furthemore, we still have references to the pip_support extra in our GitHub README and docs, despite not supporting that extra anymore:

pip support

conda-lock can also lock the dependencies.pip section of [environment.yml][envyaml], using [Poetry's][poetry] dependency solver, if installed with the pip_support extra.

We should remove all references of PIP_SUPPORT in our codebase for simplicity and clarity.

Why is this needed?

It would make the codebase a little bit simpler and clearer for new users and contributers.

What should happen?

There shouldn't be any change from a user's perspective, other than the documentation not referencing the pip_support extra anymore.

Additional Context

No response