Remove unnecessary context manager layer

Since `activate()` returns a context manager, we don't need to
construct another one.

ext/opentelemetry-ext-opentracing-shim/src/opentelemetry/ext/opentracingshim/__init__.py

@@ -156,7 +156,6 @@ def unwrap(self):
 
         return self._otel_tracer
 
-    @contextmanager
     def start_active_span(
         self,
         operation_name,
@@ -175,8 +174,7 @@ def start_active_span(
             start_time=start_time,
             ignore_active_span=ignore_active_span,
         )
-        with self._scope_manager.activate(span, finish_on_close) as scope:
-            yield scope
+        return self._scope_manager.activate(span, finish_on_close)
 
     def start_span(
         self,