Remove INT64_HEX_FORMAT and UINT64_HEX_FORMAT

Remove INT64_HEX_FORMAT and UINT64_HEX_FORMAT

These were introduced (commit efdc7d74753) at the same time as we were
moving to using the standard inttypes.h format macros (commit
a0ed19e0a9e).  It doesn't seem useful to keep a new already-deprecated
interface like this with only a few users, so remove the new symbols
again and have the callers use PRIx64.

(Also, INT64_HEX_FORMAT was kind of a misnomer, since hex formats all
use unsigned types.)

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/0ac47b5d-e5ab-4cac-98a7-bdee0e2831e4%40eisentraut.org