rust: fix clippy warning for unused import

While debug_validate_bug_on is still used, it does not need to be
imported directly, as that macro is marked with `macro_export`, making
it globally available to the crate.

(cherry picked from commit 50224f2ee5)

rust/src/modbus/detect.rs

@@ -16,7 +16,6 @@
  */
 
 use super::modbus::ModbusTransaction;
-use crate::debug_validate_bug_on;
 use lazy_static::lazy_static;
 use regex::Regex;
 use sawp_modbus::{AccessType, CodeCategory, Data, Flags, FunctionCode, Message};