fix(verify): drop unused Ast import to unbreak Rust CI

clippy -D unused-imports rejected the Ast trait import because Bool's
inherent methods cover everything called in this file. Removing Ast
restores compilation of the lib target, which had been blocking the
test and coverage jobs on main.

Author: hyperpolymath

src/verification/z3_integration.rs

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: PMPL-1.0-or-later
 
-use z3::ast::{Ast, Bool};
+use z3::ast::Bool;
 use z3::{Config, Context, Solver};
 
 use crate::game::Level;