Remove configure's check for rl_completion_append_character.

Remove configure's check for rl_completion_append_character.

The comment for PGAC_READLINE_VARIABLES says "Readline versions < 2.1
don't have rl_completion_append_character".  It seems certain that such
versions are extinct in the wild, though; for sure there are none in the
buildfarm.  Libedit has had this variable for at least twenty years too.
Also, tab-complete.c's behavior without it is quite unfriendly, since
we'll emit a space even when completion fails; but we've had no
complaints about that.

Therefore, let's assume this variable is always there, and drop the
configure check to save a few build cycles.

Discussion: https://postgr.es/m/147685.1643858911@sss.pgh.pa.us