If CONFIG_USB_DEV_PULLUP_GPIO is defined, a link error occurs because the
set_GPIO_mode() helper function is not implemented.  This function doesn't do
much except make the code a little more readable, so I just manually coded its
equivalent and removed the prototype from the header file.  It is invoked no
where else in the code.

While I was at it, I noticed that two other function prototypes in the same
header file are also neither implemented nor invoked anywhere, so I removed them
as well.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>