Remove old OpenTelemetry Semantic Convention dependency from third_party_dependencies

[michaelpri10]

In #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.