Simplify vlc_rand_bytes on Windows

Use system preferred RNG and skip the unnecessary initialization steps (in this context, which is definitely not security related) The vlc_rand_bytes function is now about 55x faster when called repeatedly in a loop See https://learn.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom for more details

Signed-off-by: Steve Lhomme robux4@ycbcr.xyz