debugger: don't set the `repl.prompt` string

It wasn't doing anything, and actually due to
3ae0b17c76f693dd2e68a46f78c7dc7f595b33c6, it was causing
the readline `prompt()` function to be overwritten
which throws an error in the REPL shortly after.
src: remove `node_isolate` from source

fix #6899
node_internals: add missing env-inl.h include