Remove current outdated implementation of interface types

[alexcrichton]

There's been a small amount of discussion on Zulip about what to do about interface types. I'd like to move that discussion here and formally propose that we take @tschneidereit's idea and do the following:

• Remove all support in wasmtime for interface types
• Parse known interface types section names and warn/error that support is not available at this time
• Link to Support for Interface Types in wasmtime API #677 for discussion about progress towards implementing interface types in wasmtime