Get rid of the "use_modules" workaround

This doesn't seem to be needed anymore with newer rust version.
As long as one symbol from the crate is used, all exported function
from the crate are available.

The reason why some symbols in some module were gone was a bug in rust
that has been fixed, it seems

api/cpp/lib.rs

@@ -10,14 +10,10 @@ use std::rc::Rc;
 #[cfg(feature = "experimental")]
 pub mod platform;
 
-#[doc(hidden)]
-#[cold]
-pub fn use_modules() -> usize {
-    #[cfg(feature = "slint-interpreter")]
-    slint_interpreter::use_modules();
-    i_slint_backend_selector::use_modules();
-    i_slint_core::use_modules()
-}
+/// One need to make sure something from the crate is exported,
+/// otherwise its symbols are not going to be in the final binary
+#[cfg(feature = "slint-interpreter")]
+pub use slint_interpreter;
 
 #[no_mangle]
 pub unsafe extern "C" fn slint_windowrc_init(out: *mut WindowAdapterRcOpaque) {