doc: Update Mapping Declaration Section

Upadate ink! mapping syntax to comform
with ink v4

Description
===========
- Removed unnecessary imports.
- Updated the ink! mapping syntax.
- Removed outdated reference to
  `ink_lang::utils::initialize_contract`.

docs/intro/ink-vs-solidity.md

@@ -262,20 +262,14 @@ mapping(address => uint128) private mapName;
 
 ```rust
 //ink!
-use ink_storage::{
-    traits::SpreadAllocate,
-    Mapping,
-};
+use ink::storage::Mapping;
 
 #[ink(storage)]
-#[derive(SpreadAllocate)]
 pub struct ContractName {
     map_name: Mapping<AccountId, u128>,
 }
 ```
 
-When using a map in ink!, `ink_lang::utils::initialize_contract` must be used in the constructor. See [here](https://use.ink/datastructures/mapping) for more details.
-
 ### `mapping usage`
 
 ```c++