usb: gadget: cleanup: fix unused variable and function warnings

usb: gadget: cleanup: fix unused variable and function warnings

Remove unused variables and functions to fix following
build warnings:

  CC      drivers/usb/gadget/configfs.o
drivers/usb/gadget/configfs.c: In function ‘gadgets_make’:
drivers/usb/gadget/configfs.c:1710:6: warning: unused variable ‘err’ [-Wunused-variable]
  int err;
      ^
drivers/usb/gadget/configfs.c:1709:27: warning: unused variable ‘attr’ [-Wunused-variable]
  struct device_attribute *attr;
                           ^
drivers/usb/gadget/configfs.c:1708:28: warning: unused variable ‘attrs’ [-Wunused-variable]
  struct device_attribute **attrs;
                            ^
drivers/usb/gadget/configfs.c: In function ‘gadgets_drop’:
drivers/usb/gadget/configfs.c:1774:27: warning: unused variable ‘attr’ [-Wunused-variable]
  struct device_attribute *attr;
                           ^
drivers/usb/gadget/configfs.c:1773:28: warning: unused variable ‘attrs’ [-Wunused-variable]
  struct device_attribute **attrs;
                            ^
<snip>...
  CC      drivers/usb/gadget/function/f_mtp.o
drivers/usb/gadget/function/f_mtp.c:1219:12: warning: ‘mtp_bind_config’ defined but not used [-Wunused-function]
 static int mtp_bind_config(struct usb_configuration *c, bool ptp_config)
            ^
drivers/usb/gadget/function/f_mtp.c:1300:12: warning: ‘mtp_setup’ defined but not used [-Wunused-function]
 static int mtp_setup(void)
            ^
<snip>...
  CC      drivers/usb/gadget/function/f_accessory.o
drivers/usb/gadget/function/f_accessory.c:969:1: warning: ‘acc_function_bind’ defined but not used [-Wunused-function]
 acc_function_bind(struct usb_configuration *c, struct usb_function *f) {
 ^
drivers/usb/gadget/function/f_accessory.c:1172:12: warning: ‘acc_bind_config’ defined but not used [-Wunused-function]
 static int acc_bind_config(struct usb_configuration *c)
            ^

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>