Remove __device__ from CUDA MR-based device allocators to fix obscure "host

function called from host device function" warning that occurs when you use the
new Thrust MR-based allocators.

Reviewed-by: Allison Vacanti <alliepiper16@gmail.com>
Reviewed-by: Michał 'Griwes' Dominiak <griwes@griwes.info>

Author: brycelelbach

thrust/system/cuda/memory.h

@@ -100,13 +100,13 @@ struct allocator
 
   /*! No-argument constructor has no effect.
    */
-  __host__ __device__
+  __host__
   inline allocator() {}
 
   /*! Copy constructor has no effect.
    */
   __host__ __device__
- inline allocator(const allocator & other) : base(other) {}
+  inline allocator(const allocator & other) : base(other) {}
 
   /*! Constructor from other \p allocator has no effect.
    */