Revert "misc/spectre: add spectre index test"

This reverts CL 222978.

Reason for revert: Test is failing frequently in TryBots and builders (https://build.golang.org/log/06a29585eaa9955cf33b50b5792b3bdc42e8fbe2)

Change-Id: I53bcf789726a9cfe78b1d3c55af78873c5c61696
Reviewed-on: https://go-review.googlesource.com/c/go/+/223378
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills