Revert support for "custom children" in uppercase components

[jchavarri]

Reverts #823.

A user complained in Discord:

I am currently trying to use the current main branch of reason-react with the new React 19 changes.
Is it intended, that it is no longer possible to have multiple children in a custom react component without a fragment?

module MyComponent = {
  [@react.component]
  let make = (~children) => {
    <section> children </section>;
  };
};

<MyComponent>
  <div> {React.string("child1")} </div>
  <div> {React.string("child2")} </div>
</MyComponent>;

This is throwing the error

Error: This expression has type < children : React.element array > Js.t
       but an expression was expected of type
         < children : React.element > Js.t
       The method children has type React.element array,
       but the expected method type was React.element

To fix this error you have to wrap the two div in a fragment (<>...</>).
I guess is coming from this change? #823

I talked to @davesnx online and while we could do some introspection in the ppx to start branching off generated code depending on the kind of children (element vs something else) it seems too much magic for the upsides.