remove OpenCensus exporter requirement from the library guideline (#631)

Author: reyang

specification/library-guidelines.md

@@ -21,7 +21,6 @@ _Note to Language Library Authors:_ OpenTelemetry specification, API and SDK imp
 5. The SDK implementation should include the following exporters:
     - Jaeger.
     - Zipkin.
-    - OpenCensus.
     - Prometheus.
     - OpenTelemetry Protocol (when the protocol is specified and approved).
     - Standard output (or logging) to use for debugging and testing as well as an input for the various log proxy tools.