Remove many variants of logical relations
This MR removes the logical relations for stlc, F_mu, and F_mu_ref, and only keeps the one for the most expressive language F_mu_ref_conc.
Moreover, it puts this logical relation, together with the one for heaplang, in the logrel folder.
This MR makes porting iris-examples after changes to Iris updates much easier---the same work often had to be redone for all versions of the logical relation.
/cc @amintimany