Ladybird: Remove Web::Platform plugins for Qt in favor of LibCore

Now that the Core::EventLoop is driven by a QEventLoop in Ladybird,
we don't need to patch LibWeb with Web::Platform plugins.

This patch removes EventLoopPluginQt and TimerQt.

Note that we can't just replace the Web::Platform abstractions with
LibCore stuff immediately, since the Web::Platform APIs use
JS::SafeFunction for callbacks.

Ladybird/WebContent/CMakeLists.txt

@@ -7,11 +7,9 @@ set(WEBCONTENT_SOURCES
     ${WEBCONTENT_SOURCE_DIR}/WebContentConsoleClient.cpp
     ${WEBCONTENT_SOURCE_DIR}/WebDriverConnection.cpp
     ../EventLoopImplementationQt.cpp
-    ../EventLoopPluginQt.cpp
     ../FontPluginQt.cpp
     ../ImageCodecPluginLadybird.cpp
     ../RequestManagerQt.cpp
-    ../TimerQt.cpp
     ../Utilities.cpp
     ../WebSocketClientManagerLadybird.cpp
     ../WebSocketLadybird.cpp