cleaning: remove iostream from headers where possible (IWYU)

[romintomasetti]

This PR removes #include <iostream> where possible from header files.

Related to "Include What You Use" policy (IWYU).

Extracted from #6198 (see /pull/6198#discussion_r1345760198).

[dalg24-jenkins]

Can one of the admins verify this patch?

[dalg24]

OK to test

[dalg24]

Assuming that the CI passes

[romintomasetti]

There are 2 failing jobs.

1. Timeout on HPX
2. Failure of a Cuda build with:

   [ 99%] Linking CXX executable Kokkos_AlgorithmsUnitTest_StdSet_Team_M
   
   /var/jenkins/workspace/Kokkos/install/include/Kokkos_SIMD_Common.hpp(452): error: "Kokkos::Experimental::simd<int32_t, Kokkos::Experimental::simd_abi::avx512_fixed_size<8>>" contains a vector, which is not supported in device code
   
         detected during:
   
           instantiation of "Kokkos::Experimental::simd<T, Abi> Kokkos::abs(const Kokkos::Experimental::simd<T, Abi> &) [with T=int32_t, Abi=Kokkos::Experimental::simd_abi::avx512_fixed_size<8>]" 
   
   (452): here

   It looks like this compile issue is uncorrelated to this PR.

[cz4rs]

There are 2 failing jobs.
(...)
It looks like this compile issue is uncorrelated to this PR.

Yep, HPX timeout is a known issue and the fix for Cuda / SIMD failure is already on its way (#6485).