remove ATTRIBUTE_FIXED_PART_SIZE

Since the introduction of the CATALOG_VARLEN stuff, the fixed size of
pg_attribute is exactly sizeof(FormData_pg_attribute), so the ancient
mechanism to track the fixed size manually using
ATTRIBUTE_FIXED_PART_SIZE can be removed.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services