Drop _FORTIFY_SOURCE=2 from defines

Gentoo toolchains enable this by default. Removing this prevents spammy
warnings about the macro being redefined.

--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1213,22 +1213,6 @@
       "__STDC_FORMAT_MACROS",
     ]
 
-    if (!is_debug && !using_sanitizer &&
-        (!is_linux || !is_clang || is_official_build) &&
-        current_cpu != "s390x" && current_cpu != "s390" &&
-        current_cpu != "ppc64" && current_cpu != "ppc64" &&
-        current_cpu != "mips" && current_cpu != "mips64") {
-      # _FORTIFY_SOURCE isn't really supported by Clang now, see
-      # http://llvm.org/bugs/show_bug.cgi?id=16821.
-      # It seems to work fine with Ubuntu 12 headers though, so use it in
-      # official builds.
-      #
-      # Non-chromium code is not guaranteed to compile cleanly with
-      # _FORTIFY_SOURCE. Also, fortified build may fail when optimizations are
-      # disabled, so only do that for Release build.
-      defines += [ "_FORTIFY_SOURCE=2" ]
-    }
-
     if (is_mac || is_ios) {
       cflags_objc = [ "-Wobjc-missing-property-synthesis" ]
       cflags_objcc = [ "-Wobjc-missing-property-synthesis" ]