Remove MinGW support

The Issue
=======

For a long time now MinGW has been unsupported and unmaintained and at this point,
it looks like something that we should just leave behind and move on.


Why Remove
==========

One of the big motivations for MinGW back in the day is that it was free compared to MSVC which was licensed based.
However, now that this is no longer true we have basically stopped updating the need CMake files.
Along with the CMake files, there are several patches to the extern libs needed to make this work.  For example, see:
https://developer.blender.org/diffusion/B/browse/master/extern/carve/patches/mingw_w64.patch

If we wanted to keep MinGW then we would need to make more custom patches to the external libs and
this is not something our platform maintainers are willing to do.

For example, here is the patches needed to build python: https://github.com/Alexpux/MINGW-packages/tree/master/mingw-w64-python3

Fixes T51301

Differential Revision: https://developer.blender.org/D2648

source/blender/python/intern/bpy_rna.c

@@ -5692,7 +5692,7 @@ PyTypeObject pyrna_struct_meta_idprop_Type = {
 	NULL,                       /* struct PyMethodDef *tp_methods; */
 	NULL,                       /* struct PyMemberDef *tp_members; */
 	NULL,                       /* struct PyGetSetDef *tp_getset; */
-#if defined(_MSC_VER) || defined(FREE_WINDOWS)
+#if defined(_MSC_VER)
 	NULL, /* defer assignment */
 #else
 	&PyType_Type,                       /* struct _typeobject *tp_base; */
@@ -6267,7 +6267,7 @@ static PyTypeObject pyrna_prop_collection_iter_Type = {
 	NULL,                       /* reprfunc tp_str; */
 
 	/* will only use these if this is a subtype of a py class */
-#if defined(_MSC_VER) || defined(FREE_WINDOWS)
+#if defined(_MSC_VER)
 	NULL, /* defer assignment */
 #else
 	PyObject_GenericGetAttr,    /* getattrofunc tp_getattro; */
@@ -6300,7 +6300,7 @@ static PyTypeObject pyrna_prop_collection_iter_Type = {
 #endif
 	/*** Added in release 2.2 ***/
 	/*   Iterators */
-#if defined(_MSC_VER) || defined(FREE_WINDOWS)
+#if defined(_MSC_VER)
 	NULL, /* defer assignment */
 #else
 	PyObject_SelfIter,          /* getiterfunc tp_iter; */
@@ -6776,7 +6776,7 @@ void BPY_rna_init(void)
 #endif
 
 	/* for some reason MSVC complains of these */
-#if defined(_MSC_VER) || defined(FREE_WINDOWS)
+#if defined(_MSC_VER)
 	pyrna_struct_meta_idprop_Type.tp_base = &PyType_Type;
 
 	pyrna_prop_collection_iter_Type.tp_iter = PyObject_SelfIter;