Remove duplicate plugin header in sources list

pepper/fullscreen_container.h sometimes appeared twice in
the sources list. This confused some other tools so it seems
like a good idea to only have it there once.

Change-Id: I02d88c68c6581eadb171ca5a5d72f75281c52717
Reviewed-on: https://chromium-review.googlesource.com/c/1293455Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#601550}

content/renderer/BUILD.gn

@@ -497,7 +497,6 @@ target(link_target_type, "renderer") {
     "p2p/socket_client_impl.h",
     "p2p/socket_dispatcher.cc",
     "p2p/socket_dispatcher.h",
-    "pepper/fullscreen_container.h",
     "peripheral_content_heuristic.cc",
     "peripheral_content_heuristic.h",
     "push_messaging/push_messaging_client.cc",