build system: delete @bazel_tools//src/conditions:windows_msys. (#649)

This condition is no longer available from Bazel 4.0.0.

BUILD.bazel

@@ -9,7 +9,6 @@ cc_library(
     copts = select({
       "@bazel_tools//src/conditions:windows": _msvc_copts,
       "@bazel_tools//src/conditions:windows_msvc": _msvc_copts,
-      "@bazel_tools//src/conditions:windows_msys": _msvc_copts,
       "//conditions:default": _gcc_copts,
     }),
 )