api: remove v2 dependencies from v3 API protos.

[htuch]

This should reduce the binary size, which is particularly important for Envoy Mobile. Looking at a
local opt build with debug symbols, I'm seeing a drop from ~400MB to ~380MB, so maybe 5% saving. @Reflejo indicates that optimized Envoy Mobile without symbols is observing ~20% improvement.

Related to #10943

Risk level: Low
Testing: bazel query deps to confirm no more v2 API deps.

Signed-off-by: Harvey Tuch htuch@google.com

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #18817 was opened by htuch.

see: more, trace.

[mattklein123]

Awesome! Check CI?

/wait

[phlax]

not sure why this has triggered a flake8 error

  File "/build/tmp/_bazel_envoybuild/b570b5ccd0454dc9af9f65ab1833764d/execroot/envoy/bazel-out/k8-fastbuild/bin/tools/code_format/python_check.runfiles/base_pip3_pypi__flake8/flake8/formatting/base.py", line 187, in _write
    sys.stdout.buffer.write(output.encode() + self.newline.encode())
AttributeError: '_io.StringIO' object has no attribute 'buffer'

ill investigate further...

[phlax]

ill investigate further...

this is the problem we are hitting - or at least the best explanation

tholo/pytest-flake8#81

unrelated to this PR, but the error message is most unhelpful

we can either downgrade flake8 package or add a workaround

@htuch in the meantime running flake8 directly, shows the problem that will unblock CI

$ pip3 install flake8
$ flake8 .
./tools/proto_format/proto_sync.py:300:62: E261 at least two spaces before inline comment
./tools/proto_format/proto_sync.py:300:63: E262 inline comment should start with '# '

[mattklein123]

CI still seems broken. :(

/wait

[mattklein123]

Yay!