tests: subsys: zbus: do not use verbose logging

This change fixes a regression on `qemu_riscv64_smp` and
`qemu_riscv32_smp` that arose because debug logging was
enabled. My guess is that subtle races exist and that
debug logging exacerbates the condidtions for those
races.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

tests/subsys/zbus/integration/prj.conf

@@ -3,7 +3,6 @@ CONFIG_ZTEST_NEW_API=y
 CONFIG_ASSERT=n
 CONFIG_LOG=y
 CONFIG_ZBUS=y
-CONFIG_ZBUS_LOG_LEVEL_DBG=y
 CONFIG_ZBUS_CHANNEL_NAME=y
 CONFIG_ZBUS_MSG_SUBSCRIBER=y
 CONFIG_HEAP_MEM_POOL_SIZE=1024