remove use_system_libwayland

It's removed from swiftshader in [1]

[1] https://swiftshader-review.googlesource.com/c/SwiftShader/+/70408

Bug: chromium:1385736
Change-Id: I6f314eca86407f7ee452d89979a3ce6c3664d819
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/114500
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>

build_overrides/wayland.gni

@@ -15,5 +15,4 @@
 import("//build/config/ozone.gni")
 
 # Dawn has no wayland third-party dir
-use_system_libwayland = true
 wayland_gn_dir = ""