Remove obsolete comment

Remove obsolete comment

This comment should probably have been moved to pg_locale_libc.c in
commit 66ac94cdc79 (2024), but upon closer examination it was already
completely obsolete then.

The first part of the comment has been obsolete since commit
85feb77aa09 (2017), which required that the system provides the
required wide-character functions.

The second part has been obsolete since commit e9931bfb751 (2024),
which eliminated code paths depending on the global LC_CTYPE setting.

Discussion: https://www.postgresql.org/message-id/flat/170308e6-a7a3-4484-87b2-f960bb564afa%40eisentraut.org