Remove Windows-related GN build configuration

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 785c8f2905eea4fea89f3e9d2d1cfa61da22370f..b0259182c56781d1607f2798bf28877605aabc87 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -153,17 +153,7 @@ declare_args() {
 # aix or one of the BSDs. If you need to check these, just check the
 # current_os value directly.
 
-if (current_os == "win") {
-  is_android = false
-  is_chromeos = false
-  is_fnl = false
-  is_ios = false
-  is_linux = false
-  is_mac = false
-  is_nacl = false
-  is_posix = false
-  is_win = true
-} else if (current_os == "mac") {
+if (current_os == "mac") {
   is_android = false
   is_chromeos = false
   is_fnl = false
@@ -236,6 +226,8 @@ if (current_os == "win") {
   is_nacl = false
   is_posix = true
   is_win = false
+} else {
+  assert(false, "Unsupported operating system")
 }
 
 # =============================================================================
@@ -540,15 +532,7 @@ set_defaults("test") {
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 
-if (is_win) {
-  # On windows we use the same toolchain for host and target by default.
-  if (is_clang) {
-    host_toolchain = "//build/toolchain/win:clang_$current_cpu"
-  } else {
-    host_toolchain = "//build/toolchain/win:$current_cpu"
-  }
-  set_default_toolchain("$host_toolchain")
-} else if (is_android) {
+if (is_android) {
   if (host_os == "linux") {
     # Use clang for the x86/64 Linux host builds.
     if (host_cpu == "x86" || host_cpu == "x64") {