Remove outdated FIXME and other commented out code

theories/tree_borrows/bor_lemmas.v

theories/tree_borrows/bor_semantics.v

theories/tree_borrows/defs.v

theories/tree_borrows/early_proofmode.v

theories/tree_borrows/expr_semantics.v

theories/tree_borrows/lang_base.v

theories/tree_borrows/logical_state.v

theories/tree_borrows/refl.v

theories/tree_borrows/steps_progress.v

theories/tree_borrows/steps_wf.v

theories/tree_borrows/tkmap_view.v

theories/tree_borrows/trees_equal/random_lemmas.v

theories/tree_borrows/trees_equal/trees_equal.v

...es/tree_borrows/trees_equal/trees_equal_asymmetric_prot.v

theories/tree_borrows/trees_equal/trees_equal_base.v

theories/tree_borrows/trees_equal/trees_equal_create_child.v

theories/tree_borrows/trees_equal/trees_equal_more_access.v

...ree_borrows/trees_equal/trees_equal_preserved_by_access.v

...orrows/trees_equal/trees_equal_preserved_by_access_base.v

theories/tree_borrows/trees_equal/trees_equal_source_read.v