Remove unused fields in ReorderBufferTupleBuf

Hi,

ReorderBufferTupleBuf is defined as follow:

/* an individual tuple, stored in one chunk of memory */
typedef struct ReorderBufferTupleBuf
{
    /* position in preallocated list */
    slist_node  node;

    /* tuple header, the interesting bit for users of logical decoding */
    HeapTupleData tuple;

    /* pre-allocated size of tuple buffer, different from tuple size */
    Size        alloc_tuple_size;

    /* actual tuple data follows */
} ReorderBufferTupleBuf;

However, node and alloc_tuple_size are not used at all. It seems an
oversight in commit a4ccc1cef5a, which introduced the generation
context and used it in logical decoding. I think we can remove them
(only on HEAD). I've attached the patch.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com