This removes the unused "argument count" field from the safepoint table
as the field was unused by now and always contained the value zero.

Also note that associating a callee's argument count with the call-site
is not compatible with tail-call support. When tail-calling a function
with a different number of arguments, the information associated with
the call-site becomes stale. The number of arguments is a property of
the callee, not of the call-site in the caller. For this reason the
field in question is not usable to support reference types in function
arguments (at least when tail-calls are also supported).

R=ahaas@chromium.org

Change-Id: If667d729267f2dd2642b755c54235cc08ca9b141
Reviewed-on: https://chromium-review.googlesource.com/c/1402548Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58969}