Eliminate nullability-only Convert nodes

[roji]

Given the following query:

await AssertQuery<Order>(isAsync, os =>
    os.Select(o => new Order { OrderDate = o.OrderDate.Value }));

we get the following tree (since OrderDate is nullable):

value(Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[Microsoft.EntityFrameworkCore.TestModels.Northwind.Order]).Select(o => new Order() {OrderDate = Convert(o.OrderDate.Value, Nullable`1)})

which makes us generate the following over-complicated SQL:

SELECT CAST(o."OrderDate" AS timestamp without time zone) AS "OrderDate" FROM "Orders" AS o

FWIW I think the old pipeline didn't have this useless cast.

[smitpatel]

https://github.com/aspnet/EntityFrameworkCore/blob/master/src/EFCore.Relational/Query/Pipeline/RelationalSqlTranslatingExpressionVisitor.cs#L543

There is already check in place to make sure if the convert node is converting from T to T? we remove it.

[smitpatel]

Fix reverted due to un-updated SQL baselines.