[RUST] Excess structs generated when using descriminators

[denumerate]

Description

When generating rust code based on an openapi spec that uses discriminators and mapped objects, excess structs are created that are confusing to the user.

openapi-generator version

Version 5.1.0

OpenAPI declaration file content or url

The openapi spec below

components:
  schemas:
    Data:
      description: The event payload.
      type: object
      discriminator:
        propertyName: type_name
        mapping:
          UserAuthentication: UserAuthentication

    UserAuthentication:
      required:
        - type_name
      description: User's authentication data.
      type: object
      properties:
        type_name:
          description: The name of this type.
          type: string
          example: UserAuthentication
        certificate:
          type: string

results in both of the following files
data.rs:

#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(tag = "typename")]
pub enum Data {
  rename="UserAuthentication")]
    UserAuthentication {
        #[serde(rename = "certificate", skip_serializing_if = "Option::is_none")]
        certificate: Option<String>,
    },
}

user_authentication.rs:

#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct UserAuthentication {
    /// The name of this type.
    #[serde(rename = "type_name")]
    pub type_name: String,
    #[serde(rename = "certificate", skip_serializing_if = "Option::is_none")]
    pub certificate: Option<String>,
}

Command line used for generation

	docker run --rm \
		-v ".:." \
		openapitools/openapi-generator-cli:v5.1.0 generate \
		-i api/openapi.yaml \
		-g rust \
		-p packageName=dataexport-models \
		-p packageVersion='0.1.0' \
		-o crates/dataexport-models \
		--ignore-file-override=openapi-generator-ignore-rust

Steps to reproduce

• Put the open api spec in an api directory
• mkdir crates
• run the above docker run command.

Related issues/PRs

This spec is based on the work in #3895.

Suggest a fix/enhancement

If either the structs were not generated or used inside the enums the resulting code would be much easier to use.