lib: remove _debugger.js

The file no longer works after the removal of the --debug/--debug-brk
switches in commit 47f8f746 ("src: remove support for --debug".)

This commit also removes several tests that still referenced the
old debugger but were either unit-testing its internals or passing
for the wrong reason (like expecting an operation to fail, which
it did because the debugger is gone.)

PR-URL: https://github.com/nodejs/node/pull/12495


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>