Revert "Add missing padding from MemoryChunk struct"

This reverts commit df0f4fe.  It turns out the problem which was causing
the 32-bit ARM and PPC animals to fail was due to a MAXALIGN problem in
slab.c.  This was fixed by d5ee4db.  The padding that was added in
df0f4fe would only do anything on machines where uint64 was not aligned
to 8 bytes.  The 32-bit machines which were failing are not in that
category, so revert this commit.

Discussion: https://postgr.es/m/3209100.1661787561@sss.pgh.pa.us

Author: david-rowley

src/include/utils/memutils_memorychunk.h

@@ -111,14 +111,6 @@ typedef struct MemoryChunk
 {
 #ifdef MEMORY_CONTEXT_CHECKING
 	Size		requested_size;
-#define MEMORYCHUNK_RAWSIZE		(SIZEOF_SIZE_T + 8)
-#else
-#define MEMORYCHUNK_RAWSIZE		8
-#endif							/* MEMORY_CONTEXT_CHECKING */
-
-	/* ensure proper alignment by adding padding if needed */
-#if (MEMORYCHUNK_RAWSIZE % MAXIMUM_ALIGNOF) != 0
-	char		padding[MAXIMUM_ALIGNOF - MEMORYCHUNK_RAWSIZE % MAXIMUM_ALIGNOF];
 #endif
 
 	/* bitfield for storing details about the chunk */