[flang][cuda] Set cudadevice interfaces as public by default (llvm#127229)

Remove the PRIVATE statement from cudadevice as it hides any interfaces
from the used __cuda_device module.

Author: clementval

flang/module/__cuda_device.f90

@@ -11,7 +11,4 @@
 module __cuda_device
 implicit none
 
-  ! Set PRIVATE by default to explicitly only export what is meant
-  ! to be exported by this MODULE.
-
 end module