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/team.go

@@ -235,7 +235,7 @@ func IsReservedTeamName(s string) bool {
 }
 
 func IsValidTeamName(s string) bool {
-	if !IsValidAlphaNum(s) {
+	if !isValidAlphaNum(s) {
 		return false
 	}