SystemMonitor: Return icon display data as GUI::Icons

And delete the generic icon member which has been dormant since
switching to FileIconProvider. Fixes icon column not being properly
painted as icon cells.

Userland/Applications/SystemMonitor/ProcessModel.h

@@ -144,6 +144,5 @@ private:
     HashMap<int, NonnullOwnPtr<Thread>> m_threads;
     NonnullOwnPtrVector<CpuInfo> m_cpus;
     Vector<int> m_tids;
-    RefPtr<Gfx::Bitmap> m_generic_process_icon;
     RefPtr<Core::File> m_proc_all;
 };