[Onion Soup] Remove dead memory usage code in BlinkPlatformImpl.

Nothing calls these methods, and this complicated cache is never used.
The cache also doesn't make sense since these APIs are only used on
the main thread so there's no reason to have locks.

BUG=561879

Review URL: https://codereview.chromium.org/1671383003

Cr-Commit-Position: refs/heads/master@{#374083}
3 files changed