usb: gadget: configfs: fix unused variable warnings

Delete unused variables 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:1708:6: warning: unused variable 'err' [-Wunused-variable]
  int err;
      ^
drivers/usb/gadget/configfs.c:1707:27: warning: unused variable 'attr' [-Wunused-variable]
  struct device_attribute *attr;
                           ^
drivers/usb/gadget/configfs.c:1706:28: warning: unused variable 'attrs' [-Wunused-variable]
  struct device_attribute **attrs;
                            ^
drivers/usb/gadget/configfs.c: In function 'gadgets_drop':
drivers/usb/gadget/configfs.c:1772:27: warning: unused variable 'attr' [-Wunused-variable]
  struct device_attribute *attr;
                           ^
drivers/usb/gadget/configfs.c:1771:28: warning: unused variable 'attrs' [-Wunused-variable]
  struct device_attribute **attrs;
                            ^
--------------------

Change-Id: Idb87fdcad0b85805e497e2fa90ce63e0b1467656
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>