These don't seem to be widely used and they're not needed by
wasm-bindgen itself, so let's remove the symbols by default and
optionally in the future we can add an option to retain them.
The `wasm-bindgen` crate is effectively the only user of this crate now
that the `wasm-gc` tool has been deprecated. It's also much easier to
keep it in this repository as it's easier to sync changes to
`parity-wasm`. I'd also like to start refactoring out utilities for
managing a `parity_wasm::Module` to share between this crate and the
other CLI support code.
