src: remove support for --debug

In the 2017-04-05 meeting, the CTC agreed to remove support for the
legacy debugger in 8.0.0. This is the first step in this direction.

Refs: https://github.com/nodejs/CTC/issues/94
PR-URL: https://github.com/nodejs/node/pull/12197


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>