Remove only boost backported serialization

Signed-off-by: Omar Shrit <omar@shrit.me>

src/mlpack/core/boost_backport/CMakeLists.txt

@@ -2,19 +2,13 @@
 # Anything not in this list will not be compiled into mlpack.
 set(SOURCES
   bernoulli.hpp
-  boost_backport_serialization.hpp
   detail/bernoulli_details.hpp
   detail/polygamma.hpp
   detail/unchecked_bernoulli.hpp
   math_fwd.hpp
   policy.hpp
-  unordered_collections_load_imp.hpp
-  unordered_collections_save_imp.hpp
-  vector.hpp
   string_view.hpp
   string_view_fwd.hpp
-  collections_load_imp.hpp
-  collections_save_imp.hpp
   boost_backport_string_view.hpp
 )
 

src/mlpack/core/boost_backport/README.md

@@ -1,7 +1,6 @@
 The files in this directory are taken from Boost 1.56.0 and Boost 1.61.0 in
 order to backport:
 
- * Serialization support for unordered_map (added in Boost 1.56).
  * Trigamma and polygamma function calculation for the gamma_distribution.hpp
    file (added in Boost 1.58.0, files taken from Boost 1.61.0)