Remove Start* macros from postmaster.c to ease understanding of code

Hi,

Attached is a small patch implemented as I agree with Andres' comment
below noted while reviewing the thread
https://www.postgresql.org/message-id/flat/20240122210740.7vq5fd4woixpwx3f%40awork3.anarazel.de#6eb7595873392621d60e6b5a723941bc

I agree that its easier to not have to refer back to the macros only to
see that they're all invoking StartChildProcess(X). All invocations are
within postmaster.c. 

> @@ -561,13 +561,13 @@ static void ShmemBackendArrayAdd(Backend *bn);
>  static void ShmemBackendArrayRemove(Backend *bn);
>  #endif							/* EXEC_BACKEND */
>  
> -#define StartupDataBase()		StartChildProcess(StartupProcess)
> -#define StartArchiver()			StartChildProcess(ArchiverProcess)
> -#define StartBackgroundWriter() StartChildProcess(BgWriterProcess)
> -#define StartCheckpointer()		StartChildProcess(CheckpointerProcess)
> -#define StartWalWriter()		StartChildProcess(WalWriterProcess)
> -#define StartWalReceiver()		StartChildProcess(WalReceiverProcess)
> -#define StartWalSummarizer()	StartChildProcess(WalSummarizerProcess)
> +#define StartupDataBase()		StartChildProcess(B_STARTUP)
> +#define StartArchiver()			StartChildProcess(B_ARCHIVER)
> +#define StartBackgroundWriter() StartChildProcess(B_BG_WRITER)
> +#define StartCheckpointer()		StartChildProcess(B_CHECKPOINTER)
> +#define StartWalWriter()		StartChildProcess(B_WAL_WRITER)
> +#define StartWalReceiver()		StartChildProcess(B_WAL_RECEIVER)
> +#define StartWalSummarizer()	StartChildProcess(B_WAL_SUMMARIZER)

Not for this commit, but we ought to rip out these macros - all they do is to make it harder to understand the code.