math: remove unused variable

math: remove unused variable

useSSE41 was used inside asm implementation of floor to select between base and ss4 code path.
We intrinsified floor and left asm functions as a backup for non-sse4 systems.
This made variable unused, so remove it.

Change-Id: Ia2633de7c7cb1ef1d5b15a2366b523e481b722d9
Reviewed-on: https://go-review.googlesource.com/97935
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>