Revert "third_party: update chromite's vendored protbuf 3.13 --> 4.21.9"

This reverts commit 86480ac9b4ab507bcbafa0304dfb9b40b120cd16.

Reason for revert: http://b/293510904

Original change's description:
> third_party: update chromite's vendored protbuf 3.13 --> 4.21.9
>
> The current copy is from 2020-08 and needs updating so that protos
> can remain idiomatic (e.g., use of the "optional" keyword). Advancing
> takes advantage of other optimizations.
>
> 4.21.9 matches the current version available in the SDK, and supports
> the "optional" keyword as well as generation of type annotations.
>
> To use it, compile_build_api_proto.py is updated to pull protoc from a
> new CIPD prefix at infra/3pp/tools/protoc, which is regularly updated.
>
> The "fork" is patched to use chromite import paths, with the only
> other change a tweak to internal/api_implementation.py to ensure it
> does not look for compiled implementations from system site packages.
>
> Patchset 1: https://github.com/protocolbuffers/protobuf/tree/v21.9/python/google/protobuf
> Patchset 2: sed -i -re 's/( *)from google\.protobuf/\1from chromite.third_party.google.protobuf/'
> Patchset 3: point compile_build_api_proto to 4.21.9 at new CIPD prefix + regenerate
> Patchset 6: fix errors picked up in ./run_tests (version check, api_implementation)
> Onwards: Rebasing (regenerate at @5b48513b1), CL description updates
>
> BUG=b:293226674
> TEST=CQ
>
> Change-Id: If557e07962eee3f75d113c49b065fe9fb9cef9bc
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4718152
> Reviewed-by: Alex Klein <saklein@chromium.org>
> Commit-Queue: Trent Apted <tapted@chromium.org>
> Tested-by: Trent Apted <tapted@chromium.org>

BUG=b:293226674

Change-Id: Ifa35253466c9cc642ce432cfe4330997276eed13
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4727763
Owners-Override: Aaron Massey <aaronmassey@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
Tested-by: Aaron Massey <aaronmassey@google.com>
diff --git a/api/compile_build_api_proto_unittest.py b/api/compile_build_api_proto_unittest.py
index db61d53..a829c6d 100644
--- a/api/compile_build_api_proto_unittest.py
+++ b/api/compile_build_api_proto_unittest.py
@@ -23,10 +23,7 @@
     TODO: Investigate using protobuf.__version__ instead of hard coding a
         version in compile_build_api_proto.
     """
-    assert (
-        f"{compile_build_api_proto.PROTOC_MAJOR_VERSION}."
-        f"{compile_build_api_proto.PROTOC_VERSION}"
-    ) == protobuf.__version__, (
+    assert compile_build_api_proto.PROTOC_VERSION == protobuf.__version__, (
         "The protobuf library or compile_build_api_proto.PROTOC_VERSION has "
         "been updated, but the other has not. They must be updated together."
     )