doc: Remove unused imports in examples (#7924)

The "os" imports in the `tmp_path` and `tmpdir` fixture examples are
unused and thus have been removed to prevent confusion.

doc/en/tmpdir.rst

@@ -20,8 +20,6 @@ created in the `base temporary directory`_.
 .. code-block:: python
 
     # content of test_tmp_path.py
-    import os
-
     CONTENT = "content"
 
 
@@ -97,9 +95,6 @@ and more.  Here is an example test usage:
 .. code-block:: python
 
     # content of test_tmpdir.py
-    import os
-
-
     def test_create_file(tmpdir):
         p = tmpdir.mkdir("sub").join("hello.txt")
         p.write("content")