broker: use default fanout of 0 for config boot

Problem: the systemd unit file has to specify a large value for
the tbon.fanout attribute to make the default system instance
use a flat TBON topology.

Make the default TBON fanout 0 (flat) when Flux is booted from
config files.

Drop the tbon.fanout=256 setting in the systemd unit file.

etc/flux.service.in

@@ -11,7 +11,6 @@ ExecStart=/bin/bash -c '\
   @X_BINDIR@/flux broker \
   --config-path=@X_SYSCONFDIR@/flux/system/conf.d \
   -Scron.directory=@X_SYSCONFDIR@/flux/system/cron.d \
-  -Stbon.fanout=256 \
   -Srundir=@X_RUNSTATEDIR@/flux \
   -Sstatedir=${STATE_DIRECTORY:-/var/lib/flux} \
   -Slocal-uri=local://@X_RUNSTATEDIR@/flux/local \

src/broker/boot_config.c

@@ -30,7 +30,7 @@
 #include "topology.h"
 #include "boot_config.h"
 
-#define DEFAULT_FANOUT 2
+#define DEFAULT_FANOUT 0
 
 
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