Remove deprecated golangci linters (#2910)
Remove deprecated golangci linters (#2910)

The "deadcode", "structcheck", and "varcheck" linters are deprecated by
golangci-lint. It is recommended by that project to use "unused"
instead. The "unused" linter is already enabled, so this just removes
the deprecated linters.
open-telemetry/opentelemetry-go-contrib,open-telemetry/opentelemetry-go-contrib,open-telemetry/opentelemetry-go-contrib