Remove OpenTelemetry extensions

[delner]

ddtrace contains some dead code that amends SpanOperation with OpenTelemetry behavior. OpenTelemetry shouldn't use Datadog::Tracing directly, instead it should use OTLP, so this code is never called.

[ivoanjo]

I love PRs that just delete stuff. The change seems reasonable, so I've approved it.

I noticed we still have two files referencing opentelemetry:

./docs/UpgradeGuide.md:| `ddtrace/opentelemetry`     | `datadog/opentelemetry`     |
./Rakefile:    t.exclude_pattern = 'spec/**/{contrib,benchmark,redis,opentracer,opentelemetry,auto_instrument}/**/*_spec.rb,'\
./Rakefile:  RSpec::Core::RakeTask.new(:opentelemetry) do |t, args|
./Rakefile:    t.pattern = 'spec/datadog/opentelemetry/**/*_spec.rb'
./Rakefile:  declare '❌ 2.1 / ❌ 2.2 / ❌ 2.3 / ✅ 2.4 / ✅ 2.5 / ✅ 2.6 / ✅ 2.7 / ✅ 3.0 / ✅ 3.1 / ✅ jruby' => 'bundle exec rake spec:opentelemetry'

Finally -- I don't quite understand the description of the PR. Could you clarify what "using OTLP" is and how it looks like?

(And should we include guidelines for using OTLP in the 1.0 upgrade notes?)

[delner]

Removing other references was a good catch. Updated those.

In short, OTLP is a way for OpenTracing to send its traces directly to the Datadog agent, bypassing our tracing library entirely. This is the preferred way to utilize OpenTracing with Datadog. It's described here: https://docs.datadoghq.com/tracing/setup_overview/open_standards/#otlp-ingest-in-datadog-agent We can add documentation for this later.