Remove references to dmalloc

There are more modern tools for memory debugging, get rid of DMALLOC
crap in the source code.

I've left one reference in backend.c to prevent a merge conflict but
this can be removed later.

src/libmdb/file.c

@@ -19,10 +19,6 @@
 #include <inttypes.h>
 #include "mdbtools.h"
 
-#ifdef DMALLOC
-#include "dmalloc.h"
-#endif
-
 /*
 typedef struct {
 	int		pg_size;