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>

models/issues/dependency.go

@@ -89,12 +89,6 @@ type ErrUnknownDependencyType struct {
 	Type DependencyType
 }
 
-// IsErrUnknownDependencyType checks if an error is ErrUnknownDependencyType
-func IsErrUnknownDependencyType(err error) bool {
-	_, ok := err.(ErrUnknownDependencyType)
-	return ok
-}
-
 func (err ErrUnknownDependencyType) Error() string {
 	return fmt.Sprintf("unknown dependency type [type: %d]", err.Type)
 }