habanalabs: remove set but not used variable 'ctx'

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

drivers/misc/habanalabs/device.c: In function hpriv_release:
drivers/misc/habanalabs/device.c:45:17: warning: variable ctx set but not used [-Wunused-but-set-variable]

It is never used since commit eb7caf84 ("habanalabs:
maintain a list of file private data objects")

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>