Eliminate unused private fields and local variables. (#9875)

This is another PR towards the goal of getting a clean build with clang,
using the compiler options used in building clang-tidy.

By default, without any new -W flags, clang warns about unused local
variables and private class fields. This PR deletes the handful of these
that clang found.

I'm assuming that the class "enode" in smt_context.cpp is the one in
smt_enode.h, so that
```
parent->get_expr()
```
calls a const method with no side effects.

src/smt/smt_context.cpp

@@ -4672,7 +4672,6 @@ namespace smt {
             theory_id th_id     = l->get_id();
 
             for (enode * parent : enode::parents(n)) {
-                auto p = parent->get_expr();
                 family_id fid = parent->get_family_id();
                 if (fid != th_id && fid != m.get_basic_family_id()) {
                     if (is_beta_redex(parent, n))