In OOo times, there'd originally been efforts to allow building on Windows with
MinGW.  Later, in LO times, this has been shifted to an attempt of cross-
compiling for Windows on Linux.  That attempt can be considered abandoned, and
the relevant code rotting.

Due to this heritage, there are now three kinds of MinGW-specific code in LO:

* Code from the original OOo native Windows effort that is no longer relevant
  for the LO cross-compilation effort, but has never been removed properly.

* Code from the original OOo native Windows effort that is re-purposed for the
  LO cross-compilation effort.

* Code that has been added specifially for the LO cross-compilation effort.

All three kinds of code are removed.

(An unrelated, remaining use of MinGW is for --enable-build-unowinreg, utilizing
--with-mingw-cross-compiler, MINGWCXX, and MINGWSTRIP.)

Change-Id: I49daad8669b4cbe49fa923050c4a4a6ff7dda568
Reviewed-on: https://gerrit.libreoffice.org/34127Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>