chore: delete observability package from Go SDK (#2182)

Part 1 of #2026

This deletes the whole observability package from the Go SDK, since no
one is using it yet. We'll replace it with an interface that better
aligns with our internal otel usage patterns once that's more mature.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

go.mod

@@ -55,7 +55,6 @@ require (
 	go.opentelemetry.io/otel/metric v1.28.0
 	go.opentelemetry.io/otel/sdk v1.28.0
 	go.opentelemetry.io/otel/sdk/metric v1.28.0
-	go.opentelemetry.io/otel/trace v1.28.0
 	go.uber.org/automaxprocs v1.5.3
 	golang.org/x/exp v0.0.0-20240613232115-7f521ea00fb8
 	golang.org/x/mod v0.19.0
@@ -102,6 +101,7 @@ require (
 	github.com/stretchr/objx v0.5.2 // indirect
 	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.52.0 // indirect
 	go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp v1.26.0 // indirect
+	go.opentelemetry.io/otel/trace v1.28.0 // indirect
 	go.opentelemetry.io/proto/otlp v1.3.1 // indirect
 	golang.org/x/tools v0.22.0 // indirect
 	gotest.tools/v3 v3.5.1 // indirect