github/workflows: disable MSYS 32-bit builds

While mpv can be build for 32-bit platforms, MSYS started dropping some
of the 32-bit packages [1]. We cannot be asked to monitor and fix CI build
each time a dependency package randomly disappears [2].

Disable MSYS 32-bit builds if the support for this platforms is phased
out upstream. Sadly this will reduce our build coverage.

[1] https://www.msys2.org/news/#2023-12-13-starting-to-drop-some-32-bit-packages
[2] fff2fa3711

See also: https://github.com/msys2/MINGW-packages/issues/19747

.github/workflows/build.yml

@@ -225,9 +225,7 @@ jobs:
       fail-fast: false
       matrix:
         sys:
-          - clang32
           - clang64
-          - mingw32
           - mingw64
           - ucrt64
     defaults: