Simplify + clarify random-bytes-fetching from KeysInterface

Simplify + clarify random-bytes-fetching from KeysInterface

Due to a desire to be able to override temporary channel IDs and
onion keys, KeysInterface had two separate fetch-random-32-bytes
interfaces - an onion-key specific version which fetched 2 random
32 byte strings and a temporary-channel-id specific version.

It turns out, we never actually need to override both at once (as
creating a new channel and sending an outbound payment are always
separate top-level calls), so there's no reason to add two
functions to the interface when both really do the same thing.