Since TrapIf has been implemented on all platforms, there is no need
anymore for the old WasmTrapHelper code. This CL also removes
TrapIf-specific tests.

R=titzer@chromium.org, clemensh@chromium.org

Change-Id: Ic069598441b7bd63bde2e66f4e536abea5ecebe6
Reviewed-on: https://chromium-review.googlesource.com/452380
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43887}