PPX: remove -no-debug option

[aantron]

The -no-debug option disables PPX backtrace support (which is currently broken anyway).

In opam, -no-debug is used only by sqlexpr.

I've never heard of any complaints that backtrace support is too slow. On the contrary, everyone wants backtrace support to be restored and improved in Lwt.

I propose we make -no-debug do nothing, simplify the PPX code, and remove the option entirely in some far future version of Lwt.

[kandu]

-no-debug ease my life when hacking lwt.ppx, the generated code is mush shorter so that the generated parsetree is acceptable to be parsed by my own eyes. 😸