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.

[suztomo]

Memo: The google-cloud-spanner-executor module in the java-spanner repository also transitively depends on 49ac09d

Error:  Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.5.0:enforce (enforce) on project google-cloud-spanner-executor: 
Error:  Rule 2: org.apache.maven.enforcer.rules.dependency.RequireUpperBoundDeps failed with message:
Error:  Failed while enforcing RequireUpperBoundDeps. The error(s) are [
Error:  Require upper bound dependencies error for io.opentelemetry.semconv:opentelemetry-semconv:1.26.0-alpha [runtime] paths to dependency are:
Error:  +-com.google.cloud:google-cloud-spanner-executor:6.86.1-SNAPSHOT
Error:    +-com.google.cloud.opentelemetry:exporter-trace:0.33.0
Error:      +-io.opentelemetry.semconv:opentelemetry-semconv:1.26.0-alpha [runtime] (managed) <-- io.opentelemetry.semconv:opentelemetry-semconv:1.27.0-alpha [runtime]
Error:  ]

Because this is a new coordinates (io.opentelemetry.semconv group ID), the problem of java-spanner will be fixed after 49ac09d is released, though.