Remove useless fallthrough annotation

A fallthrough attribute after the last case is a constraint violation
in C23, and clang warns about it (not about this comment, but if we
changed it to an attribute).  Remove it.  (There was apparently never
anything after this to fall through to, even in the first commit
da07a1e8565.)

Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/76a8efcd-925a-4eaf-bdd1-d972cd1a32ff%40eisentraut.org

src/backend/postmaster/postmaster.c

@@ -4229,7 +4229,6 @@ bgworker_should_start_now(BgWorkerStartTime start_time)
 		case PM_INIT:
 			if (start_time == BgWorkerStart_PostmasterStart)
 				return true;
-			/* fall through */
 	}
 
 	return false;