Cleanup: remove unused variable `$network_exists` in `populate_network()` function #33484

Job	Run time
Determine Matrix	4s
Single Site Memcached / Test current	12m 8s
Multisite Memcached / Test current	12m 12s
Single Site Default / Test current	12m 25s
Multisite Default / Test before	12m 27s
Single Site Default / Test before	12m 17s
Single Site Memcached / Test before	11m 47s
Multisite Default / Test current	12m 23s
Multisite Memcached / Test before	12m 22s
Compare / Single Site Memcached	16s
Compare / Multisite Memcached	16s
Compare / Multisite Default	16s
Compare / Single Site Default	22s
Slack Notifications	-1s
Failed workflow tasks	0s
	1h 39m 14s

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