Remove SchemaBuilder dependency from StructArray constructors

[Rafferty97]

Summary

Three functions in array/struct_array.rs use SchemaBuilder in the course of building a StructArray. However, in all cases they just extract the fields from the schema and throw out the empty metadata. It seems it would be more straightforward to just collect the fields into a Vec directly.

I had considered that this might be intentional, perhaps in case the implementation of SchemaBuilder is ever changed, for example to collect into something other than a Vec. If so, I'd appreciate a little more context because as it stands this abstraction seems more confusing than useful.

Proposal

In the three mentioned functions, collect into a Vec<Field> directly and then .into() it into a Fields. This eliminates the dependency on SchemaBuilder, making the code easier to read in isolation.