Remove some dead code in objects_from_file

Since rust-lang#684 was merged, it is
impossible for `obj` to not start with dst - if `obj` is absolute or
contains `../` in its Path, then the path will be hashed, and the file
will still be placed under dst.

As such, this obj.starts_with(&dst) check is always true.

src/lib.rs

@@ -3839,13 +3839,6 @@ fn objects_from_files(files: &[Arc<Path>], dst: &Path) -> Result<Vec<Object>, Er
         } else {
             dst.join(file).with_extension("o")
         };
-        let obj = if !obj.starts_with(&dst) {
-            dst.join(obj.file_name().ok_or_else(|| {
-                Error::new(ErrorKind::IOError, "Getting object file details failed.")
-            })?)
-        } else {
-            obj
-        };
 
         match obj.parent() {
             Some(s) => fs::create_dir_all(s)?,