Fix copy-paste error related to the autovacuum launcher in pgstat_io.c

Fix copy-paste error related to the autovacuum launcher in pgstat_io.c

Autovacuum launchers perform no WAL IO reads, but pgstat_tracks_io_op()
was tracking them as an allowed combination for the "init" and "normal"
contexts.

This caused the "read", "read_bytes" and "read_time" attributes of
pg_stat_io to show zeros for the autovacuum launcher rather than NULL.
NULL means that a combination of IO object, IO context and IO operation
has no meaning for a backend type.  Zero is the same as telling that a
combination is relevant, and that WAL reads are possible in an
autovacuum launcher, but it is not relevant.

Copy-pasto introduced in a051e71e28a1.

Author: Ranier Vilela <ranier.vf@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAEudQAopEMAPiUqE7BvDV+x2fUPmKmb9RrsaoDR+hhQzLKg4PQ@mail.gmail.com