Minor cleanup in calling interceptors for loading properties.

R=mvstanton@chromium.org

Review URL: https://chromiumcodereview.appspot.com/82743003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18054 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Drop unused Assembler::set_external_target_at() method.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/85793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18055 ce2b1a6d-e550-0410-aec6-3dcde31c8c00