Remove the default --tag flag from client commands

[JakeDawkins]

client:codegen was defaulting to --tag=current when not specifying a flag. That's because there was a default set. It didn't matter if you set another tag in your config, because the CLI flags took precedence over any other configs.

This PR just removes the default flag, not the default tag. Hopefully this should be non-breaking in any way since the current tag fallback is already set other places as a last-resort item.

fallbacks:

• https://github.com/apollographql/apollo-tooling/blob/master/packages/apollo-language-server/src/config/config.ts#L147
• https://github.com/apollographql/apollo-tooling/blob/master/packages/apollo-language-server/src/engine/operations/schemaTagInfo.ts#L4
• https://github.com/apollographql/apollo-tooling/blob/master/packages/apollo-language-server/src/schema/providers/engine.ts#L54

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.

[trevor-scheer]

Woo generated type check!