Allow plug-ins to be built by cmake (#3752)

* Remove references to AVX code.

* Allow plugins to be built by cmake

src/common/math.h

@@ -12,7 +12,6 @@
 #include <cmath>
 #include <algorithm>
 #include <utility>
-#include "avx_helpers.h"
 
 namespace xgboost {
 namespace common {
@@ -25,10 +24,6 @@ XGBOOST_DEVICE inline float Sigmoid(float x) {
   return 1.0f / (1.0f + expf(-x));
 }
 
-inline avx::Float8 Sigmoid(avx::Float8 x) {
-  return avx::Sigmoid(x);
-}
-
 /*!
  * \brief Do inplace softmax transformaton on start to end
  *