Remove 32-bit MinGW from CI matrix

32-bit MinGW is being phased out:

https://www.msys2.org/news/#2023-12-13-starting-to-drop-some-32-bit-packages

With eigen3 and glog packages now removed, Ceres Solver no longer can be
built using stock MinGW. Therefore, we remove the support altogether.

Change-Id: I42d4ca3d15e7f3a5ddfa2eae409db7d87aed99ea

.github/workflows/windows.yml

@@ -15,11 +15,9 @@ jobs:
       fail-fast: true
       matrix:
         build_type: [Release]
-        sys: [mingw32, mingw64]
+        sys: [mingw64]
         lib: [shared, static]
         include:
-          - sys: mingw32
-            env: i686
           - sys: mingw64
             env: x86_64