Tests: Bluetooth: Fix stack overflow for tester

Increase value of `CONFIG_BTTESTER_BTP_CMD_THREAD_STACK_SIZE` for
nrf54L15 board to avoid stack overflow.

Remove the config in the Mesh overlay as it's was also added for nrf54L15.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>