configure: Don't require nonfree for nvenc

As the nvEncodeApi.h header is now MIT licensed, this can be dropped.
The loaded CUDA and NVENC libraries are part of the nvidia driver, and
thus count as system libraries.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

configure

@@ -4076,7 +4076,6 @@ die_license_disabled nonfree cuda
 die_license_disabled nonfree libfaac
 die_license_disabled nonfree libfdk_aac
 die_license_disabled nonfree libnpp
-die_license_disabled nonfree nvenc
 die_license_disabled nonfree openssl
 
 die_license_disabled version3 libopencore_amrnb