Drop the redundant readline configuration check from CI

Drop the redundant readline configuration check from CI

.check_readline_setup grepped build/config.h to confirm the requested
libedit/readline backend was actually selected.  It dates to 2020
(bc1c0d2ef4), the autotools era, when several backends
(readline/editline/libedit) were selectable and autoconf's detection
could silently fall back to a different one or to none.

main builds only with meson now, where "line" is a strict feature
option: -Dline=enabled makes libedit a hard dependency (configure fails
if it is missing) and -Dline=disabled guarantees HAVE_LIBEDIT is never
set.  Both assertions the check makes are therefore tautological -- the
silent-misdetection failure mode it guarded against can no longer occur.

Assisted-by: Claude:claude-opus-4-8