Remove the StackDriver trace exporter (#452)

Vendor exporters needed to be hosted outside this codebase.

Additionally, remove the http-stackdriver example.

Resolves #447

Partially addresses #449

Co-authored-by: Rahul Patel <rahulpa@google.com>

Author: MrAlias

.gitignore

@@ -13,8 +13,6 @@ example/grpc/client/client
 example/grpc/server/server
 /example/http/client/client
 /example/http/server/server
-/example/http-stackdriver/client/client
-/example/http-stackdriver/server/server
 /example/jaeger/jaeger
 /example/namedtracer/namedtracer
 /example/prometheus/prometheus