Remove the mingw32 builds

mingw32 has been dead for years, and is missing huge swaths of the
Windows APIs.  strerror_s() was the straw that broke the camels
back here, but there's a lot of hackery for it.

I'm not going to go through and actually remove the build support
yet since I may want to do the 1.2 SyncTERM release with it (though
likely not), but it's not long for this world.

.gitlab-ci.yml

@@ -28,15 +28,6 @@ include:
       os: 'freebsd'
       platform: 'amd64'
       tagname: 'FreeBSD'
-  - local: '.gitlab-ci-unix.yml'
-    inputs:
-      os: 'mingw32'
-      platform: 'x86'
-      tagname: 'FreeBSD'
-      cross_platform: 'yes'
-      build_flags: 'AR=mingw32-ar AS=mingw32-as CC=mingw32-gcc CXX=mingw32-g++ RANLIB=mingw32-ranlib WINDRES=mingw32-windres RELEASE=1'
-      no_javascript: 'yes'
-      has_curses: 'no'
   - local: '.gitlab-ci-unix.yml'
     inputs:
       os: 'raspian'