Bluetooth: samples: Remove unnecessary main stack configuration

Remove unnecessary main stack configuration in bluetooth samples.
The HCI power control sample does not need to set the main stack this
low, and setting it causes restriction on what the application can
change in menuconfig. A change in the entropy driver causes stack
overflow.
The HCI uart sample sets the main stack to the default size.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>