samples: net: mqtt: Remove CONFIG_BT_MAX_CMD_LEN assignment
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 50678b03cb46 ("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 2975ca07543a ("Bluetooth: Kconfig:
Rename CONFIG_BLUETOOTH_* to CONFIG_BT_*").

Remove the assignment.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,punitvara/zephyr,finikorg/zephyr,mbolivar/zephyr,mbolivar/zephyr,mbolivar/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,mbolivar/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,mbolivar/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,kraj/zephyr,zephyriot/zephyr,kraj/zephyr,finikorg/zephyr,punitvara/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyriot/zephyr,finikorg/zephyr,kraj/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,zephyriot/zephyr,zephyriot/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr