Remove test-case workarounds for ancient libedit versions.

Remove test-case workarounds for ancient libedit versions.

This reverts some hacks added in d33a81203 and cd69ec66c.
At the time the concern was the already-ancient version of
libedit shipped in Debian 10 (Buster).  That platform is
now two years past EOL, so desupporting it for PG 18 seems
fine.  (Also, if anyone is really hot to keep testing it,
they can use SKIP_READLINE_TESTS to skip this test.)

We might have to reconsider if any animals still in the
buildfarm don't like this, but the best way to find out
is to try it.

Anton Melnikov

Discussion: https://postgr.es/m/CAGRrpzZU48F2oV3d8eDLr=4TU9xFH5Jt9ED+qU1+X91gMH68Sw@mail.gmail.com