Remove the ospath compat shim from webkitpy

Remove the ospath compat shim from webkitpy
https://bugs.webkit.org/show_bug.cgi?id=78170

Reviewed by Eric Seidel.

We no longer need this compat shim now that we don't support Python 2.5.

* Scripts/webkitpy/common/checkout/scm/git.py:
* Scripts/webkitpy/common/checkout/scm/svn.py:
* Scripts/webkitpy/common/system/filesystem.py:
(FileSystem.relpath):
* Scripts/webkitpy/common/system/filesystem_mock.py:
(MockFileSystem.relpath):
* Scripts/webkitpy/common/system/ospath.py: Removed.
* Scripts/webkitpy/common/system/ospath_unittest.py: Removed.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107145 268f45cc-cd09-0410-ab3c-d52691b4dbfc