[receiver/sapm] Remove dependency on opencensus

[atoulme]

Component(s)

receiver/sapm

Describe the issue you're reporting

➜ sapmexporter git:(rm-opencensus-goleak) go mod why go.opencensus.io

go.opencensus.io

github.com/open-telemetry/opentelemetry-collector-contrib/exporter/sapmexporter
github.com/signalfx/sapm-proto/client
go.opencensus.io/stats
go.opencensus.io/stats.test
go.opencensus.io/trace
go.opencensus.io/internal
go.opencensus.io

This dependency is no longer warranted. It should be removed so we no longer import opencensus into contrib.

[github-actions]

Pinging code owners:

• receiver/sapm: @atoulme

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