Remove use_system_ssl GYP flag

tools/gn/secondary/crypto/ssl/flags.gni

Index: tools/gn/secondary/crypto/ssl/flags.gni
diff --git a/tools/gn/secondary/crypto/ssl/flags.gni b/tools/gn/secondary/crypto/ssl/flags.gni
index aac6e96c8bbc4592a293346c4b9fd2ffc5d5566c..6891c49ea585cddecde7d24787e402ff2d12dcdc 100644
--- a/tools/gn/secondary/crypto/ssl/flags.gni
+++ b/tools/gn/secondary/crypto/ssl/flags.gni
@@ -8,9 +8,6 @@ declare_args() {
   # Use OpenSSL instead of NSS. This is used for Android and is experimental
   # in other cases (see http://crbug.com/62803).
   use_openssl = false
-
-  # Use the built-in system SSL library rather than our tree's libnss/OpenSSL.
-  use_system_ssl = false
 }
 
 if (is_linux && !use_openssl) {