[runtime] Remove useless DELETE builtin.

[runtime] Remove useless DELETE builtin.

The DELETE builtin calls through to %DeleteProperty anyway, so we
can as well skip the builtin completely and always call into the
runtime directly.  Also add different entries depending on whether
calling code is in sloppy or strict/strong mode.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1291973002

Cr-Commit-Position: refs/heads/master@{#30148}