pgsql: 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: /messages/by-id/147685.1643858911@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4b0e37faaf5ffe1552a07438323341c6394890da

Modified Files
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config/programs.m4 | 22 ++--------------------
configure | 41 +----------------------------------------
src/bin/psql/tab-complete.c | 10 +---------
src/include/pg_config.h.in | 4 ----
src/tools/msvc/Solution.pm | 1 -
5 files changed, 4 insertions(+), 74 deletions(-)