The signature is already set on the {WasmGraphBuilder}, so we don't
need to pass it again to the {Build*} functions.

R=titzer@chromium.org

Change-Id: I21e93f78211e84a9960b3fd5dffc1c94778b85e5
Reviewed-on: https://chromium-review.googlesource.com/602034Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47162}