Remove build flags for runtime/trace support (#1498)

The minimum version of Go this project supports is 1.14 meaning that all
supported versions of Go support the runtime/trace package. Remove
specific build overrides for versions of Go prior to 1.11 that are not
supported by this project.

sdk/trace/tracer.go

@@ -16,6 +16,7 @@ package trace // import "go.opentelemetry.io/otel/sdk/trace"
 
 import (
 	"context"
+	rt "runtime/trace"
 
 	"go.opentelemetry.io/otel/internal/trace/parent"
 	"go.opentelemetry.io/otel/trace"
@@ -65,7 +66,15 @@ func (tr *tracer) Start(ctx context.Context, name string, options ...trace.SpanO
 		}
 	}
 
-	ctx, end := startExecutionTracerTask(ctx, name)
-	span.executionTracerTaskEnd = end
+	ctx, span.executionTracerTaskEnd = func(ctx context.Context) (context.Context, func()) {
+		if !rt.IsEnabled() {
+			// Avoid additional overhead if
+			// runtime/trace is not enabled.
+			return ctx, func() {}
+		}
+		nctx, task := rt.NewTask(ctx, name)
+		return nctx, task.End
+	}(ctx)
+
 	return trace.ContextWithSpan(ctx, span), span
 }