staging: silicom: remove driver

The driver hasn't been cleaned up and it doesn't look like anyone is
working on it anymore (including the original author). So remove it.
If someone wants to work on cleaning the driver up and moving it out of
staging, this commit can be reverted.

In addition, since this removes the CONFIG_NET_VENDOR_SILICOM config
symbol, remove the symbol from all defconfig files that reference it.

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Cc: Daniel Cotey <puff65537@bansheeslibrary.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>