Delete leftover build script

This script is a leftover from when we had a monolithic image for all
ecosystems.

Now that we use a specific docker image just for python, that dockerfile
uses the parameterized version of this script, called
`build_for_version`.

You can see more historical context in this commit: 3e99935

I grep'd around and couldn't find any remaining usages of the `build`
script, so let's nuke it.

python/README.md

@@ -9,9 +9,8 @@ We rely on `pyenv` to manage Python's versions.
 Updating the list of known versions might be tricky, here are the steps:
 1. Update `pyenv` version in our [`Dockerfile`](https://github.com/dependabot/dependabot-core/blob/main/Dockerfile), you may use commit hash if new `pyenv` version is not released yet
 2. Then, update `pyenv global` version in `Dockerfile`, we always use the latest (and the greatest) Python version there is
-3. The next step is to update our [`build`](https://github.com/dependabot/dependabot-core/blob/main/python/helpers/build) script, bump the latest Python version there
-4. Now, update the list of known Python version in [`python_versions.rb`](https://github.com/dependabot/dependabot-core/blob/main/python/lib/dependabot/python/python_versions.rb)
-5. The last step is to tweak our tests, if required. The easiest way to determine which one to tweak is just by analyzing the failing output
+3. Now, update the list of known Python version in [`python_versions.rb`](https://github.com/dependabot/dependabot-core/blob/main/python/lib/dependabot/python/python_versions.rb)
+4. The last step is to tweak our tests, if required. The easiest way to determine which one to tweak is just by analyzing the failing output
 
 [Example PR](https://github.com/dependabot/dependabot-core/pull/3440) that does all these things.