math: delete unused function sqrtC

It appears to be a trivial dreg. Unreferenced. Gone.

Change-Id: I4a5ceed48e84254bc8a07fdb04487a18a0edf965
Reviewed-on: https://go-review.googlesource.com/20122
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/math/sqrt.go

@@ -142,7 +142,3 @@ func sqrt(x float64) float64 {
 	ix = q>>1 + uint64(exp-1+bias)<<shift // significand + biased exponent
 	return Float64frombits(ix)
 }
-
-func sqrtC(f float64, r *float64) {
-	*r = sqrt(f)
-}