supply: remove pre-installing mercurial

[arjun024]

Edited Fri 8/26 with more details

What does this change address?

• There's no documentation that suggests that while installing from
  a remote url via a VCS like hg, the mercurial package should be
  pre-installed. Pip documentation just states that it requires a working
  executable to be available, which already exists on the stack.

‣ docker run --init -it cloudfoundry/cflinuxfs3 bash -c "hg --version"
Mercurial Distributed SCM (version 4.5.3)

• When fixtures/mercurial is built by this branch's buildpack,
  we can see in the log python-hglib (which was the package referred to in
  the testdata by hg clone url) is installed.

Successfully installed Flask-2.2.2 ... python-hglib-2.6.2+2.1e7a64588ab0 ...

• Git history suggests that pre-installing mercurial via pip install mercurial (link)
  came into this buildpack from the original heroku buildpack fork.
  Heroku has since removed it. See Remove redundant Mercurial install step heroku/heroku-buildpack-python#1111

What does this change NOT address?

• This change does not address why running an app with mercurial present in
  the requirements.txt fails with the error pointing to a non-existent include path to
  Python.h even after include location is set via CFLAGS in 028a7b6.
  See CI log. The timing of this failure appearing in CI suggests that it's related to the
  package using the new setuptools version as a transitive dependency. See python3.8.12 - fatal error: Python.h: No such file or directory #574
  This has to be separately investigated.

[arjun024]

Regarding the What does this change NOT address? section:
Some things I tried - uncommented and put into one diff: ed26b63