Remove the default padding for RSA keyrings

The default was added when Node.js only supported OAEP-SHA-1-MFG1-SHA1,
but in v12 all the SHA flavors were added.

Having a default of SHA-1 when SHA-256 is available is a bad customer experience.
Since these values must agree on the keyring used to encrypt and the keyring to decrypt,
and this value will advance as the world changes,
having a default for this is a bad idea.

Locate the RSA keyring implementation and its tests, then trace where the default padding is selected for encryption and decryption. Confirm the expected behavior for callers that do not specify padding and verify that the keyring requires matching explicit SHA settings.

Written by the indexing model from the issue text.