fix(rm): remove unused is_writable function and simplify logic

The `is_writable` function was unused and has been removed to clean up the codebase. The logic has been simplified by keeping only the necessary `is_writable_metadata` function, which is now the single source of truth for checking writable permissions. This change reduces code duplication and improves maintainability.

Author: mattsu2020

src/uu/rm/src/rm.rs

@@ -551,27 +551,11 @@ fn is_writable_metadata(metadata: &Metadata) -> bool {
     (mode & 0o200) > 0
 }
 
-/// Whether the given file or directory is writable.
-#[cfg(unix)]
-fn is_writable(path: &Path) -> bool {
-    match fs::metadata(path) {
-        Err(_) => false,
-        Ok(metadata) => is_writable_metadata(&metadata),
-    }
-}
-
 #[cfg(not(unix))]
 fn is_writable_metadata(_metadata: &Metadata) -> bool {
     true
 }
 
-/// Whether the given file or directory is writable.
-#[cfg(not(unix))]
-fn is_writable(_path: &Path) -> bool {
-    // TODO Not yet implemented.
-    true
-}
-
 /// Recursively remove the directory tree rooted at the given path.
 ///
 /// If `path` is a file or a symbolic link, just remove it. If it is a