[Observability] Fix CPP observability test docker build (#34081)

CPP Observability test started failing about [2 weeks
ago](https://fusion2.corp.google.com/ci/kokoro/prod:grpc-gcp%2Ftools%2Fobservability%2Fmaster%2Fcontinuous_cpp/activity/401f3183-4097-4090-9142-d2cfe19bdf8a/log).

Turns out the `grpc/grpc` repo
[added](c73e743637 (diff-2f754321d62f08ba8392b9b168b83e24ea2852bb5d815d63e767f6c3d23c6ac5))
a `.dockerignore` file which ignores `.git` directory on `COPY`
commands. This broke the observability test docker build stage because
there was a `git submodule update` command.

But on the other hand, this `git submodule update` command in the
Observability test docker file isn't necessary and can be removed.

Adhoc test run based on this PR passed:
https://fusion2.corp.google.com/invocations/09ce384f-c077-4666-9a85-3ca890db6a80/log

tools/dockerfile/observability-test/cpp/Dockerfile

@@ -24,8 +24,7 @@ RUN apt-get update && apt-get install -y clang curl git
 WORKDIR /grpc
 COPY . .
 
-RUN git submodule update --init --recursive && \
+RUN /grpc/tools/bazel build test/cpp/interop:observability_interop_test
-  /grpc/tools/bazel build test/cpp/interop:observability_interop_test