Remove expect-fx syntax and handling

This was never fully hooked up in platforms, and the plan is to replace the need for this with doing purity-inference on normal `expect` statements.

On the other hand, fuzzing is finding some bugs caused by having a hyphenated keyword, so this is a great time to go ahead and remove it!

crates/compiler/can/src/debug/pretty_print.rs

@@ -453,7 +453,6 @@ fn expr<'a>(c: &Ctx, p: EPrec, f: &'a Arena<'a>, e: &'a Expr) -> DocBuilder<'a,
         ),
         Dbg { .. } => todo!(),
         Expect { .. } => todo!(),
-        ExpectFx { .. } => todo!(),
         Return { .. } => todo!(),
         TypedHole(_) => todo!(),
         RuntimeError(_) => todo!(),