Remove merged versionadded tags from os docs.

Doc/library/os.rst

@@ -114,10 +114,6 @@ process and user.
    automatically when an item is deleted from ``os.environ``, and when
    one of the :meth:`pop` or :meth:`clear` methods is called.
 
-   .. versionchanged:: 2.6
-      Also unset environment variables when calling :meth:`os.environ.clear`
-      and :meth:`os.environ.pop`.
-
 
 .. function:: chdir(path)
               fchdir(fd)
@@ -422,8 +418,6 @@ by file descriptors.
    Change the mode of the file given by *fd* to the numeric *mode*.  See the docs
    for :func:`chmod` for possible values of *mode*.  Availability: Unix.
 
-   .. versionadded:: 2.6
-
 
 .. function:: fchown(fd, uid, gid)
 
@@ -431,8 +425,6 @@ by file descriptors.
    and *gid*.  To leave one of the ids unchanged, set it to -1.
    Availability: Unix.
 
-   .. versionadded:: 2.6
-
 
 .. function:: fdatasync(fd)
 
@@ -803,8 +795,6 @@ Files and Directories
    affects the symlink rather than the target. See the docs for :func:`chmod`
    for possible values of *mode*.  Availability: Unix.
 
-   .. versionadded:: 2.6
-
 
 .. function:: lchown(path, uid, gid)