[m-rev.] diff: Delete unused import.

[Peter Wang]

This was breaking bootcheck at -O5 --intermod-opt when trying to compile
the stage 2 compiler. Not sure why the warning only showed up then.

---

compiler/unify_gen_construct.m:
    Delete unused import.

diff --git a/compiler/unify_gen_construct.m b/compiler/unify_gen_construct.m
index 1ba9f859b..8169df046 100644
--- a/compiler/unify_gen_construct.m
+++ b/compiler/unify_gen_construct.m
@@ -73,21 +73,20 @@
 :- import_module libs.globals.
 :- import_module libs.options.
 :- import_module ll_backend.closure_gen.
 :- import_module ll_backend.code_util.
 :- import_module ll_backend.layout.
 :- import_module ll_backend.unify_gen_deconstruct.
 :- import_module ll_backend.unify_gen_util.
 :- import_module mdbcomp.
 :- import_module mdbcomp.sym_name.
 :- import_module parse_tree.prog_type.
-:- import_module parse_tree.set_of_var.
 
 :- import_module bool.
 :- import_module cord.
 :- import_module int.
 :- import_module map.
 :- import_module pair.
 :- import_module require.
 :- import_module string.
 :- import_module term.
 :- import_module uint.
-- 
2.18.0