types: correct this for validate.validator schematype option

[vkarpov15]

Fix #14696

Summary

Ensure this has the correct type in validate.validator below.

    isActive: {
      type: Boolean,
      default: false,
      validate: {
        validator(v: any) {
          expectAssignable<User>(this);
          return !v || this.name === 'super admin';
        }
      }
    }

I made 2 changes to support this:

1. passing EnforcedDocType down to ValidateFn via ValidateOpts and SchemaValidator
2. Removing LegacyAsyncValidateFn. For some reason, LegacyAsyncValidateFn breaks these tests, and I have no idea why. It has something to do with the done function. Either way, LegacyAsyncValidateFn is no longer necessary because we removed support for validators that receive a callback in Mongoose 6, so I think it is safe to remove.

Examples