Revert "Allow replacing trace SDK (#1234)"

[tbpg]

This reverts commit fc3822b.

This is causing a break in Google Cloud Functions, which depends on master when running in GOPATH mode.

See GoogleCloudPlatform/functions-framework-go#56.

Here is the error:

src/github.com/cloudevents/sdk-go/v2/extensions/distributed_tracing_extension.go:161:3: cannot use span (type trace.Span) as type *trace.Span in return argument:
	*trace.Span is pointer to interface, not interface; Error ID: 1093f764

[tbpg]

cc @nilebox @dashpole

[nilebox]

@tbpg It seems that you're using the HEAD revision for some reason? The breaking change wasn't released yet AFAICT.
Can't you pin the dependency tag to v0.22.5?

@dashpole if we can come up with a backward compatible solution that would be preferred over the reverted change.
We can't expect every library to migrate to the latest OpenCensus release.

[tbpg]

In GOPATH mode, you need some extra dependency/vendor management to depend on a specific version. We don't have that at the moment.