samples: net: mqtt: Remove CONFIG_BT_MAX_CMD_LEN assignment

This symbol started out as CONFIG_BLUETOOTH_MAX_CMD_LEN, which was
removed in commit 50678b03 ("Bluetooth: Reuse HCI command buffers
for the command response").

The non-existing symbol in the assignment was then renamed to
CONFIG_BT_MAX_CMD_LEN by commit 2975ca07 ("Bluetooth: Kconfig:
Rename CONFIG_BLUETOOTH_* to CONFIG_BT_*").

Remove the assignment.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>