Remove JSBackend from config.toml
Remove JSBackend from config.toml

JSBackend is implied when building the emscripten backend, and not available for the standard llvm backend.  This commit also puts the example config in sync with the defaults in src/bootstrap/native.rs
aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust