Remove dead code found in executable

Context

While experimenting with dune 3.0, I played with a new option (executables_implicit_empty_intf true) (see issue #2008 (closed)). This option makes dune generate an empty mli for all executable that don't have one already. The MR removes all deadcode found with that experiment.

Spotted 2 tests that were not running and plugged them back.

Manually testing the MR

Rely on the CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
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