[sdk-platform-java] Remove old OpenTelemetry Semantic Convention dependency from third_party_dependencies

In https://github.com/googleapis/sdk-platform-java/pull/3402, the OpenTelemetry Semantic Convention dependency is being updated from `io.opentelemetry:opentelemetry-semconv` to `io.opentelemetry.semconv:opentelemetry-semconv`. However, the old package is being kept until the Pub/Sub Java client no longer depends on it. This is to track removing the old dependency once the Pub/Sub dependency is updated.