It was only still being used to store framebuffer blend constants.
Use sw::float4 instead.

Bug: b/146224130
Change-Id: Ie5660e237f4b675564d5a492f9d0a38505d07953
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43388Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>