remove default destructors & some default constructors

Another ~700 KB reduction in binary size

src/qe/mbp/mbp_arith.cpp

@@ -43,8 +43,6 @@ namespace mbp {
         imp(ast_manager& m) :
             m(m), a(m) {}
 
-        ~imp() {}
-
         void insert_mul(expr* x, rational const& v, obj_map<expr, rational>& ts) {
             rational w;
             if (ts.find(x, w))

src/qe/mbp/mbp_arrays.cpp

@@ -1058,7 +1058,6 @@ namespace mbp {
         scoped_ptr<contains_app> m_var;
 
         imp(ast_manager& m): m(m), a(m), m_stores(m) {}
-        ~imp() {}
 
         bool solve(model& model, app_ref_vector& vars, expr_ref_vector& lits) {
             return false;

src/qe/mbp/mbp_term_graph.cpp

@@ -187,8 +187,6 @@ class term {
         m_is_peq = is_partial_eq(to_app(m_expr));
     }
 
-    ~term() {}
-
     class parents {
         term const &t;
 

src/qe/nlqsat.cpp

@@ -558,7 +558,6 @@ namespace qe {
             vector<div>   m_divs;
         public:
             div_rewriter_cfg(nlqsat& s): m(s.m), a(s.m), m_zero(a.mk_real(0), m) {}
-            ~div_rewriter_cfg() {}
             br_status reduce_app(func_decl* f, unsigned sz, expr* const* args, expr_ref& result, proof_ref& pr) {
                 rational r1, r(1);
                 if (a.is_div(f) && sz == 2 && a.is_numeral(args[0], r1) && a.is_numeral(args[1], r) && !r.is_zero()) {