message: common: Remove unused ADIOS2_MPI_SIZE_T constant
  The `helper::Comm` encapsulation handles sending `size_t` data
  automatically.
diff: #include "adios2/helper/mpiwrap.h"
  -#include <climits> //UXXX_MAX
  -#include <cstdint> //SIZE_MAX
  -
  -#if SIZE_MAX == UINT_MAX
  -#define ADIOS2_MPI_SIZE_T MPI_UNSIGNED
  -#elif SIZE_MAX == ULONG_MAX
  -#define ADIOS2_MPI_SIZE_T MPI_UNSIGNED_LONG
  -#elif SIZE_MAX == ULLONG_MAX
  -#define ADIOS2_MPI_SIZE_T MPI_UNSIGNED_LONG_LONG
  -#else
  -#error "size_t could not be mapped to a MPI Type"
  -#endif
  -
  #endif /* ADIOS2_ADIOSMPI_H_ */
  