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
```

model/role.go

@@ -443,8 +443,8 @@ func (r *Role) Patch(patch *RolePatch) {
 func (r *Role) MergeChannelHigherScopedPermissions(higherScopedPermissions *RolePermissions) {
 	mergedPermissions := []string{}
 
-	higherScopedPermissionsMap := AsStringBoolMap(higherScopedPermissions.Permissions)
-	rolePermissionsMap := AsStringBoolMap(r.Permissions)
+	higherScopedPermissionsMap := asStringBoolMap(higherScopedPermissions.Permissions)
+	rolePermissionsMap := asStringBoolMap(r.Permissions)
 
 	for _, cp := range AllPermissions {
 		if cp.Scope != PermissionScopeChannel {
@@ -950,3 +950,11 @@ func AddAncillaryPermissions(permissions []string) []string {
 	}
 	return permissions
 }
+
+func asStringBoolMap(list []string) map[string]bool {
+	listMap := make(map[string]bool, len(list))
+	for _, p := range list {
+		listMap[p] = true
+	}
+	return listMap
+}