[runtime] Remove the redundant %_IsObject intrinsic.

[runtime] Remove the redundant %_IsObject intrinsic.

%_IsObject(foo) is equivalent to typeof foo === 'object' and has
exactly the same optimizations, so there's zero need for %_IsObject
in our code base.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1313903003

Cr-Commit-Position: refs/heads/master@{#30380}