Remove a "register" keyword from C++ code

The register keyword has no effect in C++, and it has been deprecated
in newer versions of the standard.

sql/filesort.cc

@@ -1514,7 +1514,7 @@ int merge_many_buff(Sort_param *param, uchar *sort_buffer,
 ulong read_to_buffer(IO_CACHE *fromfile, BUFFPEK *buffpek,
                      uint rec_length)
 {
-  register ulong count;
+  ulong count;
   ulong length= 0;
 
   if ((count= (ulong) MY_MIN((ha_rows) buffpek->max_keys,buffpek->count)))