simd/archsimd: delete DotProductQuadruple methods for now

simd/archsimd: delete DotProductQuadruple methods for now

The DotProductQuadruple methods are currently defined on Int8
vectors. There are some problems for that.

1. We defined a DotProductQuadrupleSaturated method, but the dot
product part does not need saturation, as it cannot overflow. It
is the addition part of VPDPBUSDS that does the saturation.
Currently we have optimization rules like

x.DotProductQuadrupleSaturated(y).Add(z) -> VPDPBUSDS

which is incorrect, in that the dot product doesn't do (or need)
saturation, and the Add is a regular Add, but we rewrite it to a
saturated add. The correct rule should be something like

x.DotProductQuadruple(y).AddSaturated(z) -> VPDPBUSDS

2. There are multiple flavors of DotProductQuadruple:
signed/unsigned × signed/unsigned, which cannot be completely
disambiguated by the type. The current naming may preclude adding
all the flavors.

For these reasons, remove the methods for now. We can add them
later with the issues addressed.

Change-Id: I549c0925afaa68c7e2cc956105619f2c1b46b325
Reviewed-on: https://go-review.googlesource.com/c/go/+/731441
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>