Remove GN GYP generator.

tools/gn/secondary/third_party/icu/BUILD.gn

Index: tools/gn/secondary/third_party/icu/BUILD.gn
diff --git a/tools/gn/secondary/third_party/icu/BUILD.gn b/tools/gn/secondary/third_party/icu/BUILD.gn
index cbe73355f93e8f158d03894821fc809d0d06f010..587f099c164145849216d1be255e98b7402aaca4 100644
--- a/tools/gn/secondary/third_party/icu/BUILD.gn
+++ b/tools/gn/secondary/third_party/icu/BUILD.gn
@@ -45,7 +45,6 @@ config("icu_code") {
 }
 
 component("icui18n") {
-  external = true
   sources = [
     "source/i18n/anytrans.cpp",
     "source/i18n/astro.cpp",
@@ -218,7 +217,6 @@ component("icui18n") {
 }
 
 component("icuuc") {
-  external = true
   sources = [
     "source/common/bmpset.cpp",
     "source/common/brkeng.cpp",
@@ -402,13 +400,11 @@ component("icuuc") {
 if (is_win) {
   # On Windows the target DLL is pre-built so just use a copy rule.
   copy("icudata") {
-    external = true
     sources = [ "windows/icudt.dll" ]
     outputs = [ "$root_out_dir/icudt.dll" ]
   }
 } else {
   static_library("icudata") {
-    external = true
     sources = [
       # These are hand-generated, but will do for now.  The linux version is an
       # identical copy of the (mac) icudt46l_dat.S file, modulo removal of the