fix: Don't reference deleted private bazel_tools bzl file (#1180)

The latest versions of Bazel have removed the
`@bazel_tools//tools/python:private/defs.bzl` file, so it can no longer
be referenced.

Work towards bazelbuild/bazel/issues/18170
diff --git a/BUILD.bazel b/BUILD.bazel
index dff608a..35a3df8 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -52,7 +52,6 @@
         "//python/pip_install:bzl",
         "//python:bzl",
         # Requires Bazel 0.29 onward for public visibility of these .bzl files.
-        "@bazel_tools//tools/python:private/defs.bzl",
         "@bazel_tools//tools/python:python_version.bzl",
         "@bazel_tools//tools/python:srcs_version.bzl",
         "@bazel_tools//tools/python:toolchain.bzl",