pgsql: Remove Start* macros in postmaster.c.

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1e285a5e137ed033d45c63364d90565a3c013b3f

Modified Files
--------------
src/backend/postmaster/postmaster.c | 44 +++++++++++++++----------------------
1 file changed, 18 insertions(+), 26 deletions(-)