docs: remove the belkin_sa from readme

Removed the belkin_sa kernel module from the usb-modem driver readme
file, since it doesn't actually exist in pkgs.

Signed-off-by: Roee Klinger <roee@thesocialproxy.com>
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>

drivers/usb-modem/README.md

@@ -25,7 +25,6 @@ machine:
       - name: cdc_ncm
       - name: net1080
       - name: cdc_subset
-      - name: belkin_sa
       - name: zaurus
       - name: cdc_wdm
       - name: rtl8153_ecm