samples: Bluetooth: Improve printing in NUS sample

Removing the message buffer and printing out the message directly.
Creating a message buffer with the size of CONFIG_BT_L2CAP_TX_MTU
creates a large empty buffer, which is expensive for the stack and
can lead to a stack overflow.
This way, the sample can be more easily integrated with
other Bluetooth functionalities, such as Bluetooth OTA.

Signed-off-by: Carlo Weidinger <carlo.weidinger@protonmail.com>