Overriding title in `getModelSchemaRef` causes duplication in OpenAPI schema

Nobody has claimed this yet.

## Steps to reproduce

Define a controller with `getModelSchemaRef(SomeModel, {title: 'TitleOverride'})`.

Controller definition

```typescript
export class UserController {
@post('/users', {
security: OPERATION_SECURITY_SPEC,
responses: {
'200': {
description: 'User',
content: {
'application/json': {
schema: {
'x-ts-type': User,
},
},
},
},
},
})
async create(
@requestBody({
content: {
'application/json': {
schema: getModelSchemaRef(NewUserRequest, {
title: 'NewUser',
}),
},
},
})
newUserRequest: NewUserRequest,
): Promise {
// implementation
}
}
```

## Current Behavior

The controller above emits schema definitions for both `NewUserRequest` and `NewUser`. While `NewUserRequest` is not referenced anywhere else in the schema.

## Expected Behavior

Only `NewUser` is present in OpenAPI schema.

## Additional information

`@loopback/repository-json-schema@2.4.2`
`@loopback/openapi-v3@3.4.1`

I presume `NewUserRequest` is generated from `getModelSchemaRef` as it should while `NewUser` comes from parsing `paramTypes` where it causes a cache miss due to overridden `title`.

A probable fix could exclude a parameter corresponding to `requestBody` from enumeration here

https://github.com/strongloop/loopback-next/blob/7f8d8356946dc236dd4daecbfae12e0a0662cf1c/packages/openapi-v3/src/controller-spec.ts#L329

## Acceptance Criteria

- [ ] Don't generate unused schema for parameter decorated with `@requestBody`

A solution I can think of is:
https://github.com/strongloop/loopback-next/blob/7f8d8356946dc236dd4daecbfae12e0a0662cf1c/packages/openapi-v3/src/controller-spec.ts#L329

should search through the content objects in the request body spec, if all contents' schemas exist in reference, then skip generating the one inferred from model ctor.

Start in packages/openapi-v3/src/controller-spec.ts around the referenced parameter enumeration and trace how requestBody schemas and inferred model constructors enter the OpenAPI components. Reproduce the title override case, then verify that a request body whose schema is already referenced does not generate an unused duplicate schema.

Written by the indexing model from the issue text.