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.)
[should be backpatched to PG18]
