Fixes trac:623: we changed SEEK_SET (and friends) to be (static const int)

in mpicxx.h a while ago, but somehow accidentally left "extern const
int" for SEEK_SET (and friends) in constants.h.  This commit removes
the extraneous "extern" versions.

This commit was SVN r20250.

The following Trac tickets were found above:
  Ticket 623 --> https://svn.open-mpi.org/trac/ompi/ticket/623

ompi/mpi/cxx/constants.h

@@ -249,12 +249,6 @@ OMPI_DECLSPEC extern const int MODE_SEQUENTIAL;
 
 OMPI_DECLSPEC extern const int DISPLACEMENT_CURRENT;
 
-#if OMPI_PROVIDE_MPI_FILE_INTERFACE && !defined(OMPI_IGNORE_CXX_SEEK) && OMPI_WANT_MPI_CXX_SEEK
-OMPI_DECLSPEC extern const int SEEK_SET;
-OMPI_DECLSPEC extern const int SEEK_CUR;
-OMPI_DECLSPEC extern const int SEEK_END;
-#endif
-
 OMPI_DECLSPEC extern const int MAX_DATAREP_STRING;
 
 // one-sided constants