PgStat_KindInfo.named_on_disk not required in shared stats

Hi all,
(Relevant folks in CC.)

While hacking on the area of pgstat_*.c, I have noticed the existence
of named_on_disk in PgStat_KindInfo, that is here to track the fact
that replication slots are a particular case in the PgStat_HashKey for
the dshash table of the stats because this kind of stats requires a
mapping between the replication slot name and the hash key.

As far as I can see, this field is not required and is used nowhere,
because the code relies on the existence of the to_serialized_name and
from_serialized_name callbacks to do the mapping.

Wouldn't it make sense to remove it?  This field is defined since
5891c7a8ed8f that introduced the shmem stats, and has never been used
since.

This frees an extra bit in PgStat_KindInfo, which is going to help me
a bit with what I'm doing with this area of the code while keeping the
structure size the same.

Thoughts?
--
Michael