Remove Dns plugin from the config

[OlivierAlbertini]

After v0.2, we added dns in the config file.

dns: { enabled: true, path: '@opentelemetry/plugin-dns' },

https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-node/src/config.ts

We should do that since It might require to add Dns plugin options.

For example, If we use zipkin exporter, dns need to ignore the http url otherwise we will have a loop.
For now, there is no feature that let's us fix this kind of behaviour...

[mayurkale22]

Remove DNS from the default list for 0.3.0 release
Release DNS as default when dns plugin is fixed
The short term fix would be to have users configure ignore urls
Long term fix is to share ignore urls with the exporter some way

per SIG