Remove `wasm-builder`'s README (#2525)
Followup of https://github.com/paritytech/polkadot-sdk/pull/2217 This PR deletes the README of the `wasm-builder` crate and moves its docs back into the rustdoc, [as requested here](https://github.com/paritytech/polkadot-sdk/pull/2217#discussion_r1406401175). (: