pg_lsn should not be marked typispreferred.

In general it's not a good idea for built-in types in the 'U' category
to be marked preferred; they could draw behavior away from user-defined
types with similarly-named operators.  pg_lsn is probably at low risk
of that right now given the lack of casts between it and other types,
but that doesn't make this marking OK.

Ordinarily we'd bump catversion when changing any predefined catalog
contents like this, but since we're past beta1, the costs of a forced
initdb seem to outweigh the benefits of guaranteed behavioral consistency.
There's not any known behavioral impact today anyway --- this is more
in the nature of being sure there's not problems in future.

Per an off-list complaint from Thomas Fanghaenel.