Modernize const handling with readline

Modernize const handling with readline

The comment

    /* On some platforms, readline is declared as readline(char *) */

is obsolete.  The casting away of const can be removed.

The const in the readline() prototype was added in GNU readline 4.2,
released in 2001.  BSD libedit has also had const in the prototype
since at least 2001.

(The commit that introduced this comment (187e865174) talked about
FreeBSD 4.8, which didn't have readline compatibility in libedit yet,
so it must have been talking about GNU readline in the base system.
This checks out, but already FreeBSD 5 had an updated GNU readline
with const.)

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/862fc1d4-9a0c-d2b6-5451-ee3dc750bcab%40eisentraut.org