Bluetooth: Mesh: Remove ADV_FLAG_PROXY_START, due to meanless

The first why introduce ADV_FLAG_PROXY_START
see here(https://github.com/zephyrproject-rtos/zephyr/pull/58826

)

The main core change is to mark ADV_FLAG_PROXY as moving to the last
enabled proxy instead of before.

But introducing the ADV_FLAG_PROXY_START flag seems meaningless.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>