Remove redundant insert method in serialize.

This seems to cause problems on recent clang, and looks totally
redundant and unused.

The const_iterator version is identical to the vector::const_iterator
one (which is a typedef thereof). Marking it private (instead of
removing) compiles fine, so this version is effectively unused even.
Merge pull request #86 from wtogami/clangfix

Remove redundant insert method in serialize.