Revert "Fix marshaling interfaces and union types (#3211)"

[StevenACoffman]

This reverts commit 3556475.

Got a report that since #3211 was merged, the generation seems to be broken.
The name of Go elements are no longer equal to the name of GQL types.
For instance, generating code with GQL type as:

union CliTargetSettingsResult @goModel(model: "github.com/raito-io/appserver/lambda/appserver/services/cli.CliTargetSettingsResult") = CliTargetSettings | PermissionDeniedError

type CliTargetSettings @goModel(model: "github.com/raito io/appserver/lambda/appserver/services/cli.TargetSettings")
...

Will result in generated code like this:

if len(graphql.CollectFields(ec.OperationContext, sel, []string{"CliTargetSettingsResult", "TargetSettings"})) == 0 {
	return graphql.Empty{}
}

As CliTargetSettingsResult and TargetSettings are Go types and not a GQL types, this will always be 0 and return graphql.Empty{}.

On top of that, this could lead to invalid JSON as a response. As Empty{} will be marshaled as an empty string I'm able to generate response data like

{"__typename":"xxxx","zzz":{"__typename":"yyy","id":"lR0QWfLtP7TsZcphexSvx","cliSettings":}}

Which panics on the JSON marshaling.

[coveralls]

coverage: 59.582% (+0.004%) from 59.578%
when pulling a95cbd9 on revert_3211
into 26da8a6 on master.

[StevenACoffman]

@krupyansky If you can try this again while addressing the unforeseen problems that this caused, I would appreciate it.