rust/modbus: derive default instead of manual impl

Cleans up a clippy lint for a trivial default impl that can be derived.

rust/src/modbus/detect.rs

@@ -46,7 +46,7 @@ lazy_static! {
     .unwrap();
 }
 
-#[derive(Debug, PartialEq)]
+#[derive(Debug, PartialEq, Default)]
 pub struct DetectModbusRust {
     category: Option<Flags<CodeCategory>>,
     function: Option<FunctionCode>,
@@ -57,20 +57,6 @@ pub struct DetectModbusRust {
     value: Option<Range<u16>>,
 }
 
-impl Default for DetectModbusRust {
-    fn default() -> Self {
-        DetectModbusRust {
-            category: None,
-            function: None,
-            subfunction: None,
-            access_type: None,
-            unit_id: None,
-            address: None,
-            value: None,
-        }
-    }
-}
-
 /// Compares a range from the alert signature to the transaction's unit_id/address/value
 /// range. If the signature's range intersects with the transaction, it is a match and true is
 /// returned.