Remove set_abi from AbiTransferable

[sivadeilra]

The set_abi trait method is defined, but never used. I'm guessing this is a leftover from some previous design.

[rylev]

This looks fine. I'm trying to remember what the purpose of set_abi originally was for. It seems that it was never used in com-rs having been borrowed from winrt-rs (now windows-rs). The mechanism is usually reserved for out params. I guess we're just requiring the user to write "raw" out params (e.g., *mut T) but if we want to add some convenience functionality to not have to write raw pointers, we could do that... However, the trend for this crate is to keep it simply and as close to raw as possible, so perhaps we just leave it as is.