chore: mini refactorings and dead code removal (#5512)

src/util/PostToThread.hpp

@@ -3,33 +3,9 @@
 #include "debug/AssertInGuiThread.hpp"
 
 #include <QCoreApplication>
-#include <QRunnable>
-#include <QThreadPool>
-
-#include <functional>
-
-#define async_exec(a) \
-    QThreadPool::globalInstance()->start(new LambdaRunnable(a));
 
 namespace chatterino {
 
-class LambdaRunnable : public QRunnable
-{
-public:
-    LambdaRunnable(std::function<void()> action)
-        : action_(std::move(action))
-    {
-    }
-
-    void run() override
-    {
-        this->action_();
-    }
-
-private:
-    std::function<void()> action_;
-};
-
 // Taken from
 // https://stackoverflow.com/questions/21646467/how-to-execute-a-functor-or-a-lambda-in-a-given-thread-in-qt-gcd-style
 // Qt 5/4 - preferred, has least allocations