Remove default value of CONFIG_PREBOOT for CONFIG_USB_STORAGE

Remove the default value "usb start" for CONFIG_USB_STORAGE as the USB
storage boot initialization is correctly managed by  distro boot command
('usb_boot' defined in include/config_distro_bootcmd.h already include
the command 'usb  start').

Fixes: 324d7799 ("Define default CONFIG_PREBOOT with right config option")
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>