math: remove asm version of Dim

math: remove asm version of Dim

Dim performance has regressed by 14% vs 1.9 on amd64.
Current pure go version of Dim is faster and,
what is even more important for performance, is inlinable, so
instead of tweaking asm implementation, just remove it.
I had to update BenchmarkDim, because it was simply reloading
constant(answer) in a loop.
Perf data below:

name   old time/op  new time/op  delta
Dim-6  6.79ns ± 0%  1.60ns ± 1%  -76.39%  (p=0.000 n=7+10)

If I modify benchmark to be the same as in this CL results are even better:

name   old time/op  new time/op  delta
Dim-6  10.2ns ± 0%   1.6ns ± 1%  -84.27%  (p=0.000 n=8+10)

Updates #21913

Change-Id: I00e23c8affc293531e1d9f0e0e49f3a525634f53
Reviewed-on: https://go-review.googlesource.com/80695
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>