Commits 7bb3102c and 3eb77eba removed the only user of the
EXTENSION_DONT_CHECK_SIZE flag, which had previously been required to
checkpoint truncated relations. Since 7bb3102c, segments have been
opened directly for synchronization without calling _mdfd_getseg(), so
it doesn't need a mode that tolerates non-final short segments. Remove
the redundant flag and associated comments.
Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/nyj4k7yur5t27rtygvx2i2lrlp6rqfvvhoiiwx4fznynksf2et%404hj2sp42alpe
48c142f78d