samples: usb: remove hid-cdc sample

We have some other simpler CDC ACM and HID samples in the tree.
Providing multiple functions is a core functionality of USB device
support, and we have other samples to demonstrate this as well. This
sample does not really provide anything unique, but has a few issues
such as unsafe HID buffer handling and calling uart_fifo_fill() outside
of the UART driver's IRQ context.
Remove the hid-cdc sample, as there is no value in this sample, and to
avoid spending time fixing and cleaning up the code.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>