Remove unused boost headers from pch.h files

[nvmkuruc]

Description of Change(s)

boost/mpl/or.hpp, boost/mpl/if.hpp, boost/mpl/and.hpp, boost/type_traits/is_signed.hpp, and boost/utility/in_place_factory.hpp are no longer used in the pxr code base and can be safely removed from all pch.h files.

Fixes Issue(s)

• Use STL type_traits instead of boost traits #2211
• Replace boost::in_place with emplace method #2291

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

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-8084

[spiffmon]

@nvmkuruc , we're going to close this in favor of rerunning our internal pch-generator script so we can be confident we'll continue to get the right results going forward. Thanks, though!