Fix comments in instr_time.h and remove an unneeded cast to int64

Hi hackers,

While working on [1], I came across what seems to be incorrect comments in
instr_time.h and an unneeded cast to int64.

Indeed, 03023a2664 represented time as an int64 on all platforms but forgot to
update the comment related to INSTR_TIME_GET_MICROSEC() and provided an incorrect
comment for INSTR_TIME_GET_NANOSEC().

Please find attached a tiny patch to correct those and, in passing, remove what
I think is an unneeded cast to int64.

[1]: https://www.postgresql.org/message-id/19E276C9-2C2B-435A-B275-8FA22222AEB8%40gmail.com

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com