Simplify template helper functions (#24570)

To avoid bloating the template helper functions, some functions could be
provided by type methods.

And the new code `data-line-type="{{.GetHTMLDiffLineType}}"` reads
better than `data-line-type="{{DiffLineTypeToStr .GetType}}"`


After the fix, screenshots (the same as before):

<details>


![image](https://user-images.githubusercontent.com/2114189/236657918-20ce01e0-1192-443e-aeb4-6b3fe1aa2102.png)


![image](https://user-images.githubusercontent.com/2114189/236657950-ee19727f-a1fc-4133-afc7-e5d1a8c1783f.png)

</details>

modules/templates/util_misc.go

@@ -122,19 +122,6 @@ func ActionContent2Commits(act Actioner) *repository.PushCommits {
 	return push
 }
 
-// DiffLineTypeToStr returns diff line type name
-func DiffLineTypeToStr(diffType int) string {
-	switch diffType {
-	case 2:
-		return "add"
-	case 3:
-		return "del"
-	case 4:
-		return "tag"
-	}
-	return "same"
-}
-
 // MigrationIcon returns a SVG name matching the service an issue/comment was migrated from
 func MigrationIcon(hostname string) string {
 	switch hostname {