Remove ctx.fragments.py.use_toolchains.

This was removed from `rules_python` in bazel-contrib/rules_python#3280.

PiperOrigin-RevId: 840486756
Change-Id: Id68530d898a44b7632d8780119c2fe8b86f46cc9

Author: gregestren

src/main/java/com/google/devtools/build/lib/rules/python/PythonConfiguration.java

@@ -108,18 +108,6 @@ public boolean buildPythonZip() {
     };
   }
 
-  /**
-   * Returns true if executable Python rules should obtain their runtime from the Python toolchain
-   * rather than legacy flags.
-   */
-  @StarlarkMethod(
-      name = "use_toolchains",
-      structField = true,
-      doc = "No-op: Python toolchains are always used.")
-  public boolean useToolchains() {
-    return true;
-  }
-
   @StarlarkMethod(
       name = "default_to_explicit_init_py",
       structField = true,