Remove Start* macros in postmaster.c.
These macros are just shorthands for calling StartChildProcess()
with the appropriate process type, and they arguably make the code
harder to understand.
Suggested-by: Andres Freund
Author: Reid Thompson
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/
e88934c02a5c66f5e8caab2025f85da6b9026d0b.camel%40crunchydata.com