The only remaining allowed occurence of a context is in WASM_TO_JS
code, which is regenerated for each instance.
This CL removes all the rest, to avoid subtle bugs where we might
forget to patch it. By renaming the BuildCallToRuntime method, we make
sure that noone accidentially calls the version which embeds a context.
For consistency, I even remove it from the WasmRunInterpreter stub,
which is never reused for new instantiations.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2708743003
Cr-Commit-Position: refs/heads/master@{#43409}