Remove use of cryptography's default_backend()

[MasterKale]

I stumbled on to the fact that the backend arg has been silently ignored since cryptography 3.1:

https://cryptography.io/en/latest/faq/#what-happened-to-the-backend-argument

cryptography stopped requiring the use of backend arguments in version 3.1 and deprecated their use in version 36.0. If you are on an older version that requires these arguments please view the appropriate documentation version or upgrade to the latest release.

Note that for forward compatibility backend is still silently accepted by functions that previously required it, but it is ignored and no longer documented.

I decided to remove all use of default_backend() that was being passed in for the backend arg to take advantage of this fact. I think it makes for more readable code.