Remove inaccurate comments (we did not drop these).

src/mlpack/methods/ann/layer/add.hpp

@@ -1,4 +1,3 @@
-// Temporarily drop.
 /**
  * @file methods/ann/layer/add.hpp
  * @author Marcus Edel

src/mlpack/methods/ann/layer/linear3d.hpp

@@ -1,4 +1,3 @@
-// Temporarily drop.
 /**
  * @file methods/ann/layer/linear3d.hpp
  * @author Mrityunjay Tripathi

src/mlpack/methods/ann/layer/noisylinear.hpp

@@ -1,4 +1,3 @@
-// Temporarily drop.
 /**
  * @file methods/ann/layer/noisylinear.hpp
  * @author Nishant Kumar