Remove redundant Assert(PgArchPID == 0); in PostmasterStateMachine

Hi,

It looks like the commit d75288fb [1] added an unnecessary
Assert(PgArchPID == 0); in PostmasterStateMachine as the if block code
gets hit only when PgArchPID == 0. PSA small patch.

[1]
commit d75288fb27b8fe0a926aaab7d75816f091ecdc27
Author: Fujii Masao <fujii@postgresql.org>
Date:   Mon Mar 15 13:13:14 2021 +0900

    Make archiver process an auxiliary process.

Regards,
Bharath Rupireddy.