pgsql: Drop support for getting signal descriptions from sys_siglist[].

Drop support for getting signal descriptions from sys_siglist[].

It appears that all platforms that have sys_siglist[] also have
strsignal(), making that fallback case in pg_strsignal() dead code.
Getting rid of it allows dropping a configure test, which seems worth
more than providing textual signal descriptions on whatever platforms
might still hypothetically have use for the fallback case.

Discussion: https://postgr.es/m/25758.1544983503@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cc92cca43162c4635e6ab5e7c61f7f8728d56d9a

Modified Files
--------------
configure | 18 ------------------
configure.in | 8 --------
src/include/pg_config.h.in | 4 ----
src/port/pgstrsignal.c | 21 +++++++++------------
4 files changed, 9 insertions(+), 42 deletions(-)