Revert "GitHub CI: Win - install whole gcc toolchain"

Seem no longer to be needed, also GCC has been used that time.
The change may very slightly (if noticable) improve CI run time but it
is consstent with instructions in wiki and also more clear.

This reverts commit 8278e15201.

.github/scripts/Windows/prepare_msys.sh

@@ -46,7 +46,8 @@ PACMAN_INSTALL='pacman -Sy --needed --noconfirm --disable-download-timeout'
 MINGW_PACKAGE_PREFIX=mingw-w64-clang-x86_64
 m=$MINGW_PACKAGE_PREFIX
 $PACMAN_INSTALL automake autoconf git make pkgconf \
-        $m-gcc-compat $m-toolchain \
+        $m-clang $m-winpthreads-git \
+        $m-gcc-compat \
         unzip zip
 $PACMAN_INSTALL $m-asciidoc \
         $m-ffmpeg \