Removed BUILD.gn reference to non-existing void_callback.h

The file void_callback.h has never existed but there was
a VoidCallback.h in third_party/WebKit until February this year
when it was deleted in a cleanup.

This might have triggered isolate step errors on trybots.

Bug: 778580

Change-Id: I659c7ec456a8c21a274d1743d701061c798c7ace
Reviewed-on: https://chromium-review.googlesource.com/c/1286811Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#600418}

third_party/blink/renderer/core/html/BUILD.gn

@@ -564,7 +564,6 @@ blink_core_sources("html") {
     "track/vtt/vtt_token.h",
     "track/vtt/vtt_tokenizer.cc",
     "track/vtt/vtt_tokenizer.h",
-    "void_callback.h",
     "window_name_collection.cc",
     "window_name_collection.h",
   ]