Remove useless try to fix warnings

[ptoffy]

getKeyPrimitives is marked as throws but is not actually throwing errors and this is yielding warnings. This fixes the warnings.

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• [N/A] I've run ./scripts/generate_boilerplate_files_with_gyb.sh and included updated generated files in a commit of this pull request

Motivation:

Currently there's a warning

/swift-crypto/Sources/_CryptoExtras/RSA/RSA_boring.swift:67:9: warning: no calls to throwing functions occur within 'try' expression

because we're using try on a non throwing function. This escalates to all of the functions that use it.
I'm not sure if removing throws from a non-actually-throwing public function is breaking, as using try on a non throwing function only yields a warning (if not in warnings-as-errors mode) but it actively is a signature change so it might still be breaking. In case we don't want to merge this as is, the warning can be removed by simply removing useless trys and we can wait for the next major version to remove the false-throwers.

Modifications:

Remove the useless try and throws where they're not needed.

Result:

The warning is gone.

[Lukasa]

Let's leave the throws on the public functions and just remove the incorrect internal trys.

[Lukasa]

Thanks @ptoffy!