Get rid of HStoreNamedField::SkipWriteBarrier().

The write barrier elimination does the right thing now, so
we can get rid of this hack.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/296023002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21384 ce2b1a6d-e550-0410-aec6-3dcde31c8c00