Breaking Change: Remove deprecated bazel/system_python.bzl alias.

Remove direct references to system_python.bzl in favor of using protobuf_deps.bzl instead. Use python/dist/system_python.bzl where it was moved in v5.27.0 if you need a direct reference.

See https://protobuf.dev/news/2024-10-02/#python-remove-alias

PiperOrigin-RevId: 686129383

bazel/system_python.bzl

@@ -1,17 +0,0 @@
-# Copyright (c) 2009-2021, Google LLC
-# All rights reserved.
-#
-# Use of this source code is governed by a BSD-style
-# license that can be found in the LICENSE file or at
-# https://developers.google.com/open-source/licenses/bsd
-
-"""Temporary alias to repository rule for using Python 3.x headers from the system."""
-
-load(
-    "//python/dist:system_python.bzl",
-    _system_python = "system_python",
-)
-
-# TODO: Temporary alias. This is deprecated and to be removed in a future
-# release. Users should now get system_python from protobuf_deps.bzl.
-system_python = _system_python