pgsql: Remove useless Assert.

Remove useless Assert.

Testing that an unsigned variable is >= 0 is pretty pointless,
as noted by Coverity and numerous buildfarm members.

In passing, add comment about new uses of "volatile" --- Coverity
doesn't much like that either, but it seems probably necessary.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1c164ef3d28dfab445a885a03e80cfd0d552f64a

Modified Files
--------------
src/backend/replication/logical/reorderbuffer.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)