fpga: dfl: afu: remove set but not used variable 'afu'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/fpga/dfl-afu-main.c: In function ‘afu_dev_destroy’:
drivers/fpga/dfl-afu-main.c:816:18: warning: variable ‘afu’
set but not used [-Wunused-but-set-variable]

It is never used, and so can be removed.

Acked-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>