.github/workflows/build.yml: drop MINGW32 build

The MINGW32 build has been failing for a while because the MSYS2
repository has dropped MINGW32 versions of various packages in commit
https://github.com/msys2/MINGW-packages/commit/6866be94df4758be12a1f76887973a3be98d4a23

I suppose nobody really uses 32 bit anymore.  I'd like to have a 32
bit test of MPD, just to verify code correctness, but on the other
hand, I don't like solving problems like this MSYS2 packaging problem.

.github/workflows/build.yml

@@ -139,7 +139,7 @@ jobs:
   build-msys2:
     strategy:
       matrix:
-        platform: ['MINGW32', 'MINGW64', 'UCRT64']
+        platform: ['MINGW64', 'UCRT64']
     defaults:
       run:
         shell: msys2 {0}