Remove OpenCensus support for Envoy 1.31

[andy-fong]

Do you have a suggestion for code improvement or tracking existing technical debt? Please describe.

Opencensus has been marked as deprecated for a while and it’s disabled by default in envoy 1.31. Config will be rejected by envoy if set.

Product Team agreed that we can remove opencensus support.

It does not appear that we have any e2e tests for Opencensus. We only have unit tests so it's not a hard blocker for upgrading to envoy 1.31. All tests should still pass without any changes.

If we need to allow this after upgrading to envoy 1.31, see the layered_runtime
section for example.

Envoy PR for this change: envoyproxy/envoy#33912

Our change log when we mentioned supprot for Opencensus.

We have api reference about it in solo.io.envoy.config.trace.v3 pkg from our doc

Eventually, we should comment out:
"envoy.tracers.opencensus": "//source/extensions/tracers/opencensus:config",
from bazel/extensions/extensions_build_config.bzl file.

Describe the solution you'd like

Do not compile in the opencensus extention.

Additional Context

No response

[nfuden]

Not required for adopting 1.31. Is a good thing for us to do

[andy-fong]

Do this in 1.19 just in case someone needs time to migrate but will document the layer_runtime config in 1.18 changelog.