Revert "[ms-cxxabi] Add a new calling convention that swaps 'this' and 'sret'"

This reverts commit r200561.

This calling convention was an attempt to match the MSVC C++ ABI for
methods that return structures by value.  This solution didn't scale,
because it would have required splitting every CC available on Windows
into two: one for methods and one for free functions.

Now that we can put sret on the second arg (r208453), and Clang does
that (r208458), revert this hack.