Remove unused header (#1960)

Including the <execution> header can cause compilation issues under certain conditions when an implicit dependency on TBB is brought in. This header was unused anyway, so it has been removed.

Authors:
  - William Hicks (https://github.com/wphicks)

Approvers:
  - Ben Frederickson (https://github.com/benfred)

URL: #1960

cpp/include/raft/core/detail/copy.hpp

@@ -16,7 +16,6 @@
 
 #pragma once
 #include <cstdio>
-#include <execution>
 #include <raft/core/cuda_support.hpp>
 #include <raft/core/device_mdspan.hpp>
 #include <raft/core/error.hpp>