usb: remove usb_pid.Kconfig file

Remove Kconfig file samples/subsys/usb/usb_pid.Kconfig added in
the commit e5cbe6a9 ("usb: cdc: Add unique PIDs for each sample")
with the motivation to have a separate ID for each sample supposedly
to be recognizable by the host.

The new USB support does not use the options, VID/PID is set
directly in the application. As a note, it is not necessary to have
unique PID for each sample, especially for the well known USB classes.

Replace the individual Kconfig options in the documentation
by a table with the references to the samples and PIDs.

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