Removed useless :lang option.

https://community.openproject.org/work_packages/17634

http://www.redmine.org/projects/redmine/repository/revisions/7902

PR: #2104
https://github.com/opf/openproject/commit/48f721200b6876d20d406589e9f3ed9a2887a61e

https://github.com/rails/rails/blame/71273430ce3f97785f688364f5495cc105cd1860/actionpack/lib/action_view/helpers/form_helper.rb#L1202

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%407902