remove unused variable `buffer`

This addresses a compiler warning from gcc-7:

    dune-uggrid/gm/gmcheck.cc:110:13: warning:
    ‘buffer’ defined but not used [-Wunused-variable]

gm/gmcheck.cc

@@ -107,7 +107,6 @@ using namespace PPIF;
 /*																			*/
 /****************************************************************************/
 
-static char buffer[4*256];                      /* general purpose text buffer			*/
 static DOUBLE hghost_overlap = 1.0;
 
 REP_ERR_FILE;