Remove ERROR_ON_UNDEFINED_SYMBOLS according to sbc100's comments

Author: hoodmane

compiler/rustc_target/src/spec/wasm32_unknown_emscripten.rs

@@ -17,8 +17,6 @@ pub fn target() -> Target {
     post_link_args.insert(
         LinkerFlavor::Em,
         vec![
-            "-s".into(),
-            "ERROR_ON_UNDEFINED_SYMBOLS=1".into(),
             "-s".into(),
             "ABORTING_MALLOC=0".into(),
             "-Wl,--fatal-warnings".into(),