Remove unnecessary assertion in postmaster.c

A code path asserted that the archiver was dead, but a check made that
impossible to happen.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACW=CYE1ars+2XyPTEPq0wQvru4c0dPZ=Nrn3EqNBkksvQ@mail.gmail.com
Backpatch-throgh: 14

src/backend/postmaster/postmaster.c

@@ -3934,7 +3934,6 @@ PostmasterStateMachine(void)
 			Assert(CheckpointerPID == 0);
 			Assert(WalWriterPID == 0);
 			Assert(AutoVacPID == 0);
-			Assert(PgArchPID == 0);
 			/* syslogger is not considered here */
 			pmState = PM_NO_CHILDREN;
 		}