Remove unused deprecated boost header include

[WardBrian]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

Since https://github.com/stan-dev/math/pull/2955, building cmdstan has been issuing the following warning:

stan/lib/stan_math/lib/boost_1.81.0/boost/config/pragma_message.hpp:24:34: note: ‘#pragma message: This header is deprecated. Use <iterator> instead.’
   24 | # define BOOST_PRAGMA_MESSAGE(x) _Pragma(BOOST_STRINGIZE(message(x)))
      |                                  ^~~~~~~
stan/lib/stan_math/lib/boost_1.81.0/boost/config/pragma_message.hpp:24:34: note: in definition of macro ‘BOOST_PRAGMA_MESSAGE’
   24 | # define BOOST_PRAGMA_MESSAGE(x) _Pragma(BOOST_STRINGIZE(message(x)))
      |                                  ^~~~~~~
stan/lib/stan_math/lib/boost_1.81.0/boost/iterator.hpp:10:1: note: in expansion of macro ‘BOOST_HEADER_DEPRECATED’
   10 | BOOST_HEADER_DEPRECATED("<iterator>")
      | ^~~~~~~~~~~~~~~~~~~~~~~

We only include this header in the multipath pathfinder implementation, and it is not necessary.

Intended Effect

Remove the deprecation warning.

How to Verify

Build cmdstan with this branch

Side Effects

Documentation

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)