Remove ffi tuple example

[Theodus]

This change reflects the changes about to be made in order to resolve ponylang/ponyc#1240

[jemc]

This isn't quite right. The setEvent example which you changed should actually still work as shown in the current tutorial - we're only talking about disallowing returning a pony tuple from an FFI call.

So it's the getEvent example (that you didn't change) which will no longer work - it would need to be returning an EGLEvent* rather than an EGLEvent in order to work.

I recommend that we just remove the getEvent example, leave the setEvent example as is, and note that FFI calls which return structs by value are not portable enough and consistent enough across compilers to be supported. Perhaps we should also change the title of "pass C structs" to "pass C structs by value" to emphasize that there is another (non-tuple) mechanism for passing and returning structs by reference.

[jemc]

Thank you!