Remove j2cl `$isInstance` methods, other unused code

[niloc132]

Several JRE emul types have $isInstance static methods, originally added so that GWT and j2cl can share JRE emul. Nothing uses this code inside of GWT itself, and it is only usable from JS if exported, or from java that isn't compiled with javac.

javaemul.internal.annotations.Wasm should also be removed, as well as Throwable.privateInitError and the override in JsException.

There are a handful of other comments with J2CL in them, but none directly point to any changes that could be made for cleanup inside of GWT.