[RFC] Removing the support for custom parser

[Halbaroth]

As you know, Alt-Ergo will use Dolmen parser as a new frontend.
The Dolmen integration has been merged on next, see #491.

In the next release 2.5.0, Dolmen will be available through the option --frontend dolmen.
We plan to set Dolmen as the default parser in the release 2.6.0 and to remove the legacy parser in the release 2.7.0.

The current version of Alt-Ergo can use custom parser through the option --add-solver.

The only use I know of this feature is the custom parser of the AB-Why3 plugin see #642.

I think we can remove this feature.

[Halbaroth]

Solved in #1251