jit: Remove {llvm-config,clang}-N configure probes.

jit: Remove {llvm-config,clang}-N configure probes.

Previously we searched for llvm-config-N and clang-N as well as the
unversioned names, and maintained a list of expected values of N.  There
doesn't seem to be any reason to think that the default llvm-config and
clang won't be good enough, and if they aren't, they can be overridden
with LLVM_CONFIG and CLANG, so let's stop maintaining that list.

The list had not been updated since LLVM 7 with no complaints, so commit
820b5af73dc probably should have just removed it when dropping support
for 7, instead of trying to be helpful by bringing it up to date with
recent version numbers.

The Meson build system didn't have that, so no change there.

Suggested-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/CA%2BhUKG%2BSOP-aR%3DYF_n0dtXGWeCy6x%2BCn-RMWURU5ySQdmeKW1Q%40mail.gmail.com