Remove some obsolete re-exports in wasmtime-jit. (#992)

These were from when wasmtime-jit was trying to present a different API;
now they're not needed.

crates/api/src/callable.rs

@@ -6,8 +6,7 @@ use crate::values::Val;
 use std::ptr;
 use std::rc::Rc;
 use wasmtime_environ::ir;
-use wasmtime_jit::InstanceHandle;
-use wasmtime_runtime::Export;
+use wasmtime_runtime::{Export, InstanceHandle};
 
 /// A trait representing a function that can be imported and called from inside
 /// WebAssembly.