Drop support for MSVCRT's %I64 format strings.

Drop support for MSVCRT's %I64 format strings.

MSVCRT predated C99 and invented non-standard placeholders for 64-bit
numbers, and then later used them in standard macros when C99
<inttypes.h> arrived.  The macros just use %lld etc when building with
UCRT, so there should be no way for our interposed sprintf.c code to
receive the pre-standard kind these days.  Time to drop the code that
parses them.

That code was in fact already dead when commit 962da900 landed, as we'd
disclaimed MSVCRT support a couple of weeks earlier in commit 1758d424,
but patch development overlapped and the history of these macros hadn't
been investigated.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/4d8b1a67-aab2-4429-b44b-f03988095939%40eisentraut.org