Remove MSVC default assignment workaround
Remove MSVC default assignment workaround

This seems to work now in VS2015, and its presence causes assumption
failures (static assertions) in dynamic.c++.

pjulien/capnproto,mologie/capnproto,jparyani/capnproto,mologie/capnproto,hntrmrrs/capnproto,jparyani/capnproto,mrdomino/capnproto,pjulien/capnproto,mologie/capnproto,pjulien/capnproto,jparyani/capnproto,mrdomino/capnproto,mrdomino/capnproto,hntrmrrs/capnproto,hntrmrrs/capnproto