Remove unused boost headers

[nvmkuruc]

Description of Change(s)

• Remove unused headers boost/ptr_container/ptr_map.hpp, boost/smart_ptr.hpp, boost/preprocessor/facilities/empty.hpp, and boost/assign.hpp from the pxr code base.
• Removed boost/preprocessor.hpp from wrapType.cpp
• Removed boost/function.hpp from several unused sites, as the boost function make_function comes from boost/python, not boost/function.hpp.

Fixes Issue(s)

• Some boost headers are unused #2243

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[tallytalwar]

Filed as internal issue #USD-8050