ci: remove mingw job #18580

Unnecessary CI builds increase the change of spurious failures, which are costly
noise. Of course, we should fix all legitimate bugs, but we also cannot
micro-manage every platform, so there needs to be a clear motivation for the CI
builds that we maintain.

Reasons against maintaining a mingw CI job:
1. The windows mingw build is slow.
2. Failures:
    - https://github.com/neovim/neovim/issues/18494
    - https://github.com/neovim/neovim/issues/18495
3. The mingw artifact is 10x bigger than the windows MSVC artifact:
   https://github.com/neovim/neovim/issues/10560
4. Our releases publish the MSVC (not mingw) artifact for Windows users:
   https://github.com/neovim/neovim/releases
5. Non-MSVCRT has limitations documented by libuv: http://docs.libuv.org/en/v1.x/process.html
   > On Windows file descriptors greater than 2 are available to the child process only if the child processes uses the MSVCRT runtime.

Closes https://github.com/neovim/neovim/issues/18551

third-party/cmake/BuildMsgpack.cmake

@@ -42,18 +42,7 @@ set(MSGPACK_CONFIGURE_COMMAND ${CMAKE_COMMAND} ${DEPS_BUILD_DIR}/src/msgpack
 set(MSGPACK_BUILD_COMMAND ${CMAKE_COMMAND} --build . --config ${CMAKE_BUILD_TYPE})
 set(MSGPACK_INSTALL_COMMAND ${CMAKE_COMMAND} --build . --target install --config ${CMAKE_BUILD_TYPE})
 
-if(MINGW AND CMAKE_CROSSCOMPILING)
-  get_filename_component(TOOLCHAIN ${CMAKE_TOOLCHAIN_FILE} REALPATH)
-  set(MSGPACK_CONFIGURE_COMMAND ${CMAKE_COMMAND} ${DEPS_BUILD_DIR}/src/msgpack
-    -DMSGPACK_BUILD_TESTS=OFF
-    -DMSGPACK_BUILD_EXAMPLES=OFF
-    -DCMAKE_INSTALL_PREFIX=${DEPS_INSTALL_DIR}
-    # Pass toolchain
-    -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN}
-    -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
-    # Hack to avoid -rdynamic in Mingw
-    -DCMAKE_SHARED_LIBRARY_LINK_C_FLAGS="")
-elseif(MSVC)
+if(MSVC)
   # Same as Unix without fPIC
   set(MSGPACK_CONFIGURE_COMMAND ${CMAKE_COMMAND} ${DEPS_BUILD_DIR}/src/msgpack
     -DMSGPACK_BUILD_TESTS=OFF