Fix issue where BQ refused to generate "complex SQL" for a select

[christopherswenson]

Fixes https://github.com/malloydata/malloy/issues/1604

Removed an error-checking condition in the compiler where if "complex SQL" was ever generated for a select it would throw an error. This was probably once reasonable when the only way you could generate complex SQL was when there were nests, but now it can also happen if you use window functions.