Remove ambiguity/contradiction in SpanContext spec. (open-telemetry#244)

Signed-off-by: Christian Neumüller <[email protected]>

specification/api-tracing.md

@@ -167,9 +167,7 @@ propagated along side of a distributed context. `SpanContext`s are immutable.
 The OpenTelemetry `SpanContext` representation conforms to the [w3c TraceContext
 specification](https://www.w3.org/TR/trace-context/). It contains two
 identifiers - a `TraceId` and a `SpanId` - along with a set of common
-`TraceFlags` and system-specific `TraceState` values. `SpanContext` is
-represented as an interface, in order to be serializable into a wider variety of
-trace context wire formats. 
+`TraceFlags` and system-specific `TraceState` values.
 
 `TraceId` A valid trace identifier is a 16-byte array with at least one
 non-zero byte.