This helper method is no longer needed since we have removed all usages
of indexes for the purpose of accessing stack spill slot.

The parameter names for FillStackSlotsWithZero in the header file is
also corrected to match the names in the implementation.

Bug: v8:9909
Change-Id: I6093f020e05701ff8e8d6362015d2e07165b2397
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1950224Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65326}