crypto: remove default encoding from DiffieHellman

getDefaultEncoding() always returns 'buffer' in Node.js 20.

In diffiehellman.js, this value is always used as input to either toBuf(), encode(), or getArrayBufferOrView(). All of these functions treat any falsy encoding just like 'buffer', so we can safely remove the calls to getDefaultEncoding().

This partially addresses:

https://github.com/nodejs/node/blob/af9b48a2f17b11b66a8b81beeaba3c408b863795/lib/internal/crypto/util.js#L77-L80

Refs: https://github.com/nodejs/node/pull/47182