remove alias.go from docs (cosmos#6498)

docs/building-modules/structure.md

@@ -46,19 +46,13 @@ x/{module}
 │   ├── params.go
 │   └── proposals.go
 ├── abci.go
-├── alias.go
 ├── genesis.go
 ├── handler.go
 ├── ...
 └── module.go
 ```
 
 - `abci.go`: The module's `BeginBlocker` and `EndBlocker` implementations (if any).
-- `alias.go`: The module's exported types, constants, and variables. These are mainly
-to improve developer ergonomics by only needing to import a single package. Note,
-there is nothing preventing developers from importing other packages from the module
-but it is recommended that `alias.go` have everything exposed that other modules
-may need.
 - `client/`: The module's CLI and REST client functionality implementation and
 testing.
 - `exported/`: The module's exported types -- typically type interfaces. If a module