No longer set a fallback value for $STACK

Since it is always set on Heroku:
https://devcenter.heroku.com/articles/buildpack-api#stacks

...and even for other environments, picking an arbitrary stack
to use does not make sense, since it's unlikely the runtimes will
run on anything but the stack for which they were compiled.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Fix typo in the `BUILD_WITH_GEO_LIBRARIES` end-of-life error message ([#1307](https://github.com/heroku/heroku-buildpack-python/pull/1307)).
+- No longer set a fallback value for `$STACK`, since it is always set on Heroku ([#1308](https://github.com/heroku/heroku-buildpack-python/pull/1308)).
 
 ## v209 (2022-03-24)
 

bin/compile

@@ -58,10 +58,6 @@ PY27="python-2.7"
 PYPY27="pypy2.7"
 PYPY36="pypy3.6"
 
-# Which stack is used (for binary downloading), if none is provided (e.g. outside of Heroku)?
-# TODO: Remove this and require that STACK be set explicitly.
-DEFAULT_PYTHON_STACK="heroku-18"
-
 # Common Problem Warnings:
 # This section creates a temporary file in which to stick the output of `pip install`.
 # The `warnings` subscript then greps through this for common problems and guides
@@ -73,9 +69,6 @@ RECOMMENDED_PYTHON_VERSION=$DEFAULT_PYTHON_VERSION
 # This installs them into the path, so we can execute them directly.
 export PATH=$PATH:$ROOT_DIR/vendor/
 
-# Set environment variables if they weren't set by the platform.
-[ ! "$STACK" ] && STACK=$DEFAULT_PYTHON_STACK
-
 # Sanitize externally-provided environment variables:
 # The following environment variables are either problematic or simply unneccessary
 # for the buildpack to have knowledge of, so we unset them, to keep the environment