ref: drop math for storage guard (#446)

Drop `add_assign_unchecked` and `sub_assign_unchecked` functions for
`StorageGuardMut`, since guard already implements `Deref` and `DerefMut`
to wrapped types.

contracts/src/utils/math/storage.rs

@@ -1,20 +1,13 @@
 //! Simple math operations missing in `stylus_sdk::storage`.
 use alloy_primitives::Uint;
-use stylus_sdk::storage::{StorageGuardMut, StorageUint};
+use stylus_sdk::storage::StorageUint;
 
+/// Adds value and assign the result to `self`, ignoring overflow.
 pub(crate) trait AddAssignUnchecked<T> {
+    /// Adds `rhs` and assign the result to `self`, ignoring overflow.
     fn add_assign_unchecked(&mut self, rhs: T);
 }
 
-impl<const B: usize, const L: usize> AddAssignUnchecked<Uint<B, L>>
-    for StorageGuardMut<'_, StorageUint<B, L>>
-{
-    fn add_assign_unchecked(&mut self, rhs: Uint<B, L>) {
-        let new_balance = self.get() + rhs;
-        self.set(new_balance);
-    }
-}
-
 impl<const B: usize, const L: usize> AddAssignUnchecked<Uint<B, L>>
     for StorageUint<B, L>
 {
@@ -24,19 +17,12 @@ impl<const B: usize, const L: usize> AddAssignUnchecked<Uint<B, L>>
     }
 }
 
+/// Subtract value and assign the result to `self`, ignoring overflow.
 pub(crate) trait SubAssignUnchecked<T> {
+    /// Subtract `rhs` and assign the result to `self`, ignoring overflow.
     fn sub_assign_unchecked(&mut self, rhs: T);
 }
 
-impl<const B: usize, const L: usize> SubAssignUnchecked<Uint<B, L>>
-    for StorageGuardMut<'_, StorageUint<B, L>>
-{
-    fn sub_assign_unchecked(&mut self, rhs: Uint<B, L>) {
-        let new_balance = self.get() - rhs;
-        self.set(new_balance);
-    }
-}
-
 impl<const B: usize, const L: usize> SubAssignUnchecked<Uint<B, L>>
     for StorageUint<B, L>
 {