Agda: Remove agda/ModalLogic.agda
Agda: Remove agda/ModalLogic.agda

This file doesn't belong to the main branch, and in fact it arguably doesn't
belong in this repo at all - it just uses the Agda metatheory library started
from Tillmann.

Old-commit-hash: 31da94b0816ff85ac9bddb0ebcd37be4291760b8

inc-lc/ilc-agda