[webkitpy] Remove unused return_exit_code argument and returned values from SCM.{add,add_list}
https://bugs.webkit.org/show_bug.cgi?id=114410

Reviewed by Ryosuke Niwa.

* Scripts/webkitpy/common/checkout/scm/git.py:
(Git.add_list): Remove optional return_exit_code argument. Remove return value.
* Scripts/webkitpy/common/checkout/scm/scm.py:
(SCM.add): Remove optional return_exit_code argument.
(SCM.add_list): Remove optional return_exit_code argument.
* Scripts/webkitpy/common/checkout/scm/svn.py:
(SVN.add_list): Remove optional return_exit_code argument. Remove return value.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148173 268f45cc-cd09-0410-ab3c-d52691b4dbfc