Remove some code for old unsupported versions of MSVC

As of d9dd406fe281d22d5238d3c26a7182543c711e74, we require MSVC 2013,
which means _MSC_VER >= 1800.  This means that conditionals about
older versions of _MSC_VER can be removed or simplified.

Previous code was also in some cases handling MinGW, where _MSC_VER is
not defined at all, incorrectly, such as in pg_ctl.c and win32_port.h,
leading to some compiler warnings.  This should now be handled better.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services