refactor: cleanup now-unreferenced proto toolchain type (#2620)

Follow-up to #2604, fixes a breaking change in v1.2.0-rc0

Note that this toolchain_type became unused in that PR. We leave behind
an alias to make this a non-breaking change.

Verified in a downstream repo that requires the toolchain_type to
register pre-built `protoc`:
https://github.com/aspect-build/toolchains_protoc/pull/50/files

---------

Co-authored-by: Richard Levasseur <[email protected]>
(cherry picked from commit f9779ee)

Author: alexeagle

python/proto/BUILD.bazel

@@ -14,5 +14,11 @@
 
 package(default_visibility = ["//visibility:public"])
 
-# Toolchain type provided by proto_lang_toolchain rule and used by py_proto_library
-toolchain_type(name = "toolchain_type")
+# Deprecated; use @com_google_protobuf//bazel/private:python_toolchain_type instead.
+# Alias is here to provide backward-compatibility; see #2604
+# It will be removed in a future release.
+alias(
+    name = "toolchain_type",
+    actual = "@com_google_protobuf//bazel/private:python_toolchain_type",
+    deprecation = "Use @com_google_protobuf//bazel/private:python_toolchain_type instead",
+)