Remove deprecated use_system_libwayland

Instead, check if wayland gn shall be used by checking whether
wayland_gn_dir is set or not.

Bug: chromium:1385736
Change-Id: Ie589e55e02a66cb06bd115d65ce96c84de9e9ad8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4169946
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
diff --git a/BUILD.gn b/BUILD.gn
index 32b6ad0..707cdb3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1132,7 +1132,7 @@
 
 if (angle_use_wayland) {
   config("angle_wayland_config") {
-    if (use_system_libwayland) {
+    if ("$wayland_gn_dir" == "") {
       libs = [
         "wayland-client",
         "wayland-egl",
@@ -1148,7 +1148,7 @@
   group("angle_wayland") {
     public_configs = [ ":angle_wayland_config" ]
 
-    if (!use_system_libwayland) {
+    if ("$wayland_gn_dir" != "") {
       # Use third-party targets
       public_deps = [
         "$wayland_gn_dir:wayland_client",
diff --git a/build_overrides/wayland.gni b/build_overrides/wayland.gni
index 76042ef..5c72e88 100644
--- a/build_overrides/wayland.gni
+++ b/build_overrides/wayland.gni
@@ -9,5 +9,3 @@
 
 # ANGLE standalone has no Wayland GN
 wayland_gn_dir = ""
-
-use_system_libwayland = true