Set all variable-length fields of pg_attribute to null on column drop

I noticed that when a column is dropped, RemoveAttributeById() clears 
out certain fields in pg_attribute, but it leaves the variable-length 
fields at the end (attacl, attoptions, and attfdwoptions) unchanged. 
This is probably harmless, but it seems wasteful and unclean, and leaves 
potentially dangling data lying around (for example, attacl could 
contain references to users that are later also dropped).

I suggest the attached patch to set those fields to null when a column 
is marked as dropped.