Kernel: Remove the per-process icon_id and sys$set_process_icon()

This was a goofy kernel API where you could assign an icon_id (int) to
a process which referred to a global shbuf with a 16x16 icon bitmap
inside it.

Instead of this, programs that want to display a process icon now
retrieve it from the process executable instead.

DevTools/UserspaceEmulator/Emulator.h

@@ -126,7 +126,6 @@ private:
     int virt$unlink(FlatPtr path, size_t path_length);
     int virt$get_process_name(FlatPtr buffer, int size);
     int virt$set_mmap_name(FlatPtr);
-    int virt$set_process_icon(int);
     int virt$gettimeofday(FlatPtr);
     int virt$clock_gettime(int, FlatPtr);
     int virt$clock_nanosleep(FlatPtr);