dfu: Drop unused prototype of dfu_trigger_reset()

After the commit 1cc03c5c ("dfu: Provide means to find difference between
dfu-util -e and -R") the dangling ptototype appeared. Remove it here.

Fixes: 1cc03c5c ("dfu: Provide means to find difference between dfu-util -e and -R")
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Lukasz Majewski <lukma@denx.de>