storage: Simplify layout of storage crate (#40124)

Due to old design choices, we had the layout of:
```
| - storage
|    - indexeddb
|        - mod.rs
|        - idb_thread.rs
|        - engines/
|    - webstorage
|        - mod.rs
|        - webstorage_thread.rs
|        - engines/
```

I merged the `*_thread` file into `mod.rs`, since `mod.rs` is pretty
empty. This should be a better layout for when more threads are
introduced.

Testing: Refactor, none needed

Signed-off-by: Ashwin Naren <arihant2math@gmail.com>

components/storage/webstorage/engines/sqlite.rs

@@ -10,8 +10,8 @@ use log::error;
 use rusqlite::Connection;
 
 use crate::shared::{DB_INIT_PRAGMAS, DB_PRAGMAS};
+use crate::webstorage::OriginEntry;
 use crate::webstorage::engines::WebStorageEngine;
-use crate::webstorage::webstorage_thread::OriginEntry;
 
 pub struct SqliteEngine {
     connection: Connection,