Clean up MinGW def file generation

Clean up MinGW def file generation

There were some leftovers from ancient ad-hoc ways to build on
Windows, prior to the standardization on MSVC and MinGW.  We don't
need to build a lib$(NAME)ddll.def (debug build, as opposed to
lib$(NAME)dll.def) for MinGW, since nothing uses that.  We also don't
need to build the regular .def file during distprep, since the MinGW
build environment is perfectly capable of creating that normally at
build time.

Discussion: https://www.postgresql.org/message-id/flat/0f9db9f8-47b8-a48b-6ccc-15b22b412316%402ndquadrant.com