[turbofan] Remove context canonicalization hack from generic lowering.

[turbofan] Remove context canonicalization hack from generic lowering.

This hack doesn't help currently, since it only applies to asm.js where
we do function context specialization and there we already have a
canonicalized context (and the CodeGenerator materializes that from the
frame).

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1177033006

Cr-Commit-Position: refs/heads/master@{#29025}