Remove PythonPreference toggle based on UV_TEST_PYTHON_PATH (#7989)

Needed for https://github.com/astral-sh/uv/pull/7934

This is some legacy logic, I think.

crates/uv-python/src/discovery.rs

@@ -328,7 +328,7 @@ fn python_executables_from_installed<'a>(
     })
     .flatten();
 
-    let from_windows = std::iter::once_with(move || {
+    let from_windows_registry = std::iter::once_with(move || {
         #[cfg(windows)]
         {
             // Skip interpreter probing if we already know the version doesn't match.
@@ -376,14 +376,14 @@ fn python_executables_from_installed<'a>(
         PythonPreference::Managed => Box::new(
             from_managed_installations
                 .chain(from_search_path)
-                .chain(from_windows),
+                .chain(from_windows_registry),
         ),
         PythonPreference::System => Box::new(
             from_search_path
-                .chain(from_windows)
+                .chain(from_windows_registry)
                 .chain(from_managed_installations),
         ),
-        PythonPreference::OnlySystem => Box::new(from_search_path.chain(from_windows)),
+        PythonPreference::OnlySystem => Box::new(from_search_path.chain(from_windows_registry)),
     }
 }
 
@@ -1522,15 +1522,6 @@ impl PythonPreference {
         }
     }
 
-    /// Return the default [`PythonPreference`], respecting the `UV_TEST_PYTHON_PATH` variable.
-    pub fn default_from_env() -> Self {
-        if env::var_os("UV_TEST_PYTHON_PATH").is_some() {
-            Self::OnlySystem
-        } else {
-            Self::default()
-        }
-    }
-
     pub(crate) fn allows_managed(self) -> bool {
         matches!(self, Self::Managed | Self::OnlyManaged)
     }