Remove configure's check for nonstandard "long long" printf modifiers.

We used to claim to support platforms using 'q' or 'I64' as the printf
length modifier for long long int, by dint of replacing snprintf with
our own code which uses the C99 standard 'll' modifier.  But that is
only adequate if we use INT64_MODIFIER only in snprintf-based calls,
not directly with the platform's native printf or fprintf.  Which
hasn't been the case for years.  We had not noticed, partially because
of inadequate test coverage, and partially because the buildfarm is
almost completely bare of machines that won't take 'll'.  The last
one seems to have been frogmouth, which was adjusted recently so that
it will take 'll'.  We might as well just give up on the pretense
that anything else works, and save ourselves some configure cycles.

Discussion: https://postgr.es/m/13103.1526749980@sss.pgh.pa.us
Discussion: https://postgr.es/m/24769.1526772680@sss.pgh.pa.us