Delete obsolete comment.

a) EIGEN_USE_NONBLOCKING_THREAD_POOL no longer exist. The non-blocking threadpool is the default (It can be disabled by the symbol EIGEN_USE_SIMPLE_THREAD_POOL).

b) An eigen_initialize.cc file is no longer needed due to recent fixes for thread safety in Eigen.
Change: 138074328

eigen.BUILD

@@ -62,8 +62,6 @@ cc_library(
         # This define (mostly) guarantees we don't link any problematic
         # code. We use it, but we do not rely on it, as evidenced above.
         "EIGEN_MPL2_ONLY",
-        # TODO(jart): Use EIGEN_USE_NONBLOCKING_THREAD_POOL but first add an
-        #             eigen_initialize.cc file and alwayslink=1.
     ],
     includes = ["."],
     visibility = ["//visibility:public"],