[ty] Remove vestigial pyvenv.cfg creation in mdtest (#18006)

Following #17991, removes some of
https://github.com/astral-sh/ruff/pull/17222 which is no longer strictly
necessary. I don't actually think it's that ugly to have around? no
strong feelings on retaining it or not.

crates/ty_test/README.md

@@ -314,9 +314,9 @@ typeshed = "/typeshed"
 
 For more details, take a look at the [custom-typeshed Markdown test].
 
-### Mocking a virtual environment
+### Mocking a Python environment
 
-Mdtest supports mocking a virtual environment for a specific test at an arbitrary location, again
+Mdtest supports mocking a Python environment for a specific test at an arbitrary location, again
 using the `[environment]` configuration option:
 
 ````markdown
@@ -326,16 +326,10 @@ python = ".venv"
 ```
 ````
 
-ty will reject virtual environments that do not have valid `pyvenv.cfg` files at the
-virtual-environment directory root (here, `.venv/pyvenv.cfg`). However, if a `pyvenv.cfg` file does
-not have its contents specified by the test, mdtest will automatically generate one for you, to
-make mocking a virtual environment more ergonomic.
-
-Mdtest also makes it easy to write Python packages to the mock virtual environment's
-`site-packages` directory using the `<path-to-site-packages>` magic path segment. This would
-otherwise be hard, due to the fact that the `site-packages` subdirectory in a virtual environment
-is located at a different relative path depending on the platform the virtual environment was
-created on. In the following test, mdtest will write the Python file to
+Mdtest also makes it easy to write Python packages to the mock environment's `site-packages`
+directory using the `<path-to-site-packages>` magic path segment. This would otherwise be hard, due
+to the fact that the `site-packages` subdirectory in an environment is located at a different
+relative path depending on the platform. In the following test, mdtest will write the Python file to
 `.venv/Lib/site-packages/foo.py` in its in-memory filesystem used for the test if the test is being
 executed on Windows, and `.venv/lib/python3.13/site-packages/foo.py` otherwise:
 

crates/ty_test/src/lib.rs

@@ -170,7 +170,6 @@ fn run_test(
 
     let mut typeshed_files = vec![];
     let mut has_custom_versions_file = false;
-    let mut has_custom_pyvenv_cfg_file = false;
 
     let test_files: Vec<_> = test
         .files()
@@ -196,9 +195,8 @@ fn run_test(
                 }
             } else if let Some(python_path) = python_path {
                 if let Ok(relative_path) = full_path.strip_prefix(python_path) {
-                    if relative_path.as_str() == "pyvenv.cfg" {
-                        has_custom_pyvenv_cfg_file = true;
-                    } else {
+                    // Construct the path to the site-packages directory
+                    if relative_path.as_str() != "pyvenv.cfg" {
                         let mut new_path = SystemPathBuf::new();
                         for component in full_path.components() {
                             let component = component.as_str();
@@ -256,16 +254,6 @@ fn run_test(
         }
     }
 
-    if let Some(python_path) = python_path {
-        if !has_custom_pyvenv_cfg_file {
-            let pyvenv_cfg_file = python_path.join("pyvenv.cfg");
-            let home_directory = SystemPathBuf::from(format!("/Python{python_version}"));
-            db.create_directory_all(&home_directory).unwrap();
-            db.write_file(&pyvenv_cfg_file, format!("home = {home_directory}"))
-                .unwrap();
-        }
-    }
-
     let configuration = test.configuration();
 
     let settings = ProgramSettings {