COMP: Remove 'python_install_chmod_library' external project install step

Since the transition to CMake'ified python, this extra step is not
required anymore because the installed file has the expected
permissions.

Because the timestamp of the library is not systematically modified, it
will avoid the systematic recompilation of all project linking against
python library and speed up the overall MacOSX developer experience.

Before the update:

[...]
-- Installing: /path/to/python-install/lib/python2.7/zipfile.py
Performing python_install_chmod_library step for 'python'
Performing python_customPython_configure step for 'python'
Completed 'python'
$ ls -als python-install/lib/
total 3688
0 drwxr-xr-x 5 kitware staff 170 Dec 17 16:00 .
0 drwxr-xr-x 6 kitware staff 204 Dec 17 16:00 ..
3688 -rwxr-xr-x 1 kitware staff 1886288 Dec 17 16:00 libpython2.7.dylib
0 drwxr-xr-x 5 kitware staff 170 Dec 17 16:00 pkgconfig
0 drwxr-xr-x 234 kitware staff 7956 Dec 17 16:00 python2.7

After the update:

[...]
-- Installing: /path/to/python-install/lib/python2.7/zipfile.py
[ 18%] Performing python_customPython_configure step for 'python'
[ 18%] Completed 'python'
$ ls -als python-install/lib/
total 3688
0 drwxr-xr-x 5 kitware staff 170 Dec 17 16:02 .
0 drwxr-xr-x 6 kitware staff 204 Dec 17 16:02 ..
3688 -rwxr-xr-x 1 kitware staff 1886288 Dec 17 16:02 libpython2.7.dylib
0 drwxr-xr-x 5 kitware staff 170 Dec 17 16:02 pkgconfig
0 drwxr-xr-x 234 kitware staff 7956 Dec 17 16:02 python2.7

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22784 3bd1e089-480b-0410-8dfb-8563597acbee