Drop thenable coercion cache

R=dslomov@chromium.org
BUG=372788
LOG=Y

Review URL: https://codereview.chromium.org/281753004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21301 ce2b1a6d-e550-0410-aec6-3dcde31c8c00