Cleanup model/utils.go (#17945)

* Cleanup model/utils.go

- Removed some functions which were unsued. (The unused linter was somehow failing to catch this).
- Moved some functions inside other packages.
- Removed two duplicate instances of the same function - RemoveDuplicateStrings and UniqueStrings.
- Moved some regexes to global vars to prevent compilation every time.

```release-note
NONE
```

* fix lint

```release-note
NONE
```

* bring back unused exception

```release-note
NONE
```

* fix tests

```release-note
NONE
```

* Address review comments

```release-note
NONE
```

api4/role.go

@@ -137,7 +137,7 @@ func patchRole(c *Context, w http.ResponseWriter, r *http.Request) {
 			}
 		}
 
-		*patch.Permissions = model.UniqueStrings(*patch.Permissions)
+		*patch.Permissions = model.RemoveDuplicateStrings(*patch.Permissions)
 	}
 
 	if c.App.Srv().License() != nil && isGuest && !*c.App.Srv().License().Features.GuestAccountsPermissions {