Remove obsolete free-space checks from MarkCompactCollector.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/17840004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15343 ce2b1a6d-e550-0410-aec6-3dcde31c8c00