When the `PropertyDiscoveryConvention` is removed, then also skip call to `ValidatePropertyMapping(model, logger)` in ModelValidator

Converting a Database First EF6 project to EF Core. The existing database is the arbiter of truth, NOT the POCOs. Model is fully defined using Fluent API and so have removed the `PropertyDiscoveryConvention` in `ConfigureConventions()` but `ModelValidator` insists that every read/write property be mapped or tagged with `Ignore` or `[NotMapped]` attribute. POCOs contain literally thousands of such properties used for functional or front end purposes that have nothing to do with the model/database (for example, hundreds of Flag type enums associated with read/write boolean properties).

Only run the `ValidatePropertyMapping(model, logger)` method in ModelValidator if `PropertyDiscoveryConvention` is present in the Model Builder's Conventions collection.