Revert "Add missing padding from MemoryChunk struct"

Revert "Add missing padding from MemoryChunk struct"

This reverts commit df0f4feef.  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 d5ee4db0e.  The padding that was added in
df0f4feef 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