Removed substitutions for base64url decoding

Node handles base64url decoding natively: https://github.com/joyent/node/blob/master/src/string_bytes.cc#L154
The ":" to "/" didn't really make sense in a context of the spec or the encode function. Was this a typo that went uncaught due to the native decoding of any remaining underscores?
Probably, yes, but my main point is that it doesn't need to be done at all :) Node can decode both base64 variants fine, bun only encodes to the default base64 encoding.