Revert: [wimp] Turn multithreading on for wimp. (#191424)

Reverts: [[wimp] Turn multithreading on for
wimp.](https://github.com/flutter/flutter/pull/190626)

Initiated by: @bkonyi

Reason for reverting: [failing Linux linux_web_engine_tests on
tree](https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20linux_web_engine_tests/9022).

Original PR Author: @eyebrowsoffire

Reviewed By: @mdebbar

The original PR description is provided below:

After https://github.com/flutter/flutter/pull/190048, we no longer have
multthreading issues with wimp, so we should turn multithreading on.

This addresses https://github.com/flutter/flutter/issues/178749