Remove dead code identified by deadcode tool (#37271)

Ran [`deadcode`](https://pkg.go.dev/golang.org/x/tools/cmd/deadcode)
(`-test ./...`) to find functions, methods and error types unreachable
from any call path (including tests), and removed the truly-dead ones.

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

services/repository/files/content.go

@@ -32,11 +32,6 @@ const (
 	ContentTypeSubmodule ContentType = "submodule" // submodule content type (submodule)
 )
 
-// String gets the string of ContentType
-func (ct *ContentType) String() string {
-	return string(*ct)
-}
-
 type GetContentsOrListOptions struct {
 	TreePath                 string
 	IncludeSingleFileContent bool // include the file's content when the tree path is a file

services/repository/files/tree.go

@@ -26,12 +26,6 @@ type ErrSHANotFound struct {
 	SHA string
 }
 
-// IsErrSHANotFound checks if an error is a ErrSHANotFound.
-func IsErrSHANotFound(err error) bool {
-	_, ok := err.(ErrSHANotFound)
-	return ok
-}
-
 func (err ErrSHANotFound) Error() string {
 	return fmt.Sprintf("sha not found [%s]", err.SHA)
 }