Fix duplicate typedefs

Similar to 615b4c69, this removes a duplicate (and dead) typedef. This
unblocks compilation on older versions of GCC.

src/include/cdb/tupser.h

@@ -29,7 +29,6 @@
 #undef TUPSER_SCRATCH_SPACE
 #define VARLEN_SCRATCH_SIZE 500
 
-typedef struct ChunkSorterEntry ChunkSorterEntry;
 typedef struct MotionConn MotionConn;
 
 /*