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.

Libraries/LibCore/ProcessStatisticsReader.h

@@ -77,7 +77,6 @@ struct ProcessStatistics {
     size_t amount_clean_inode;
     size_t amount_purgeable_volatile;
     size_t amount_purgeable_nonvolatile;
-    int icon_id;
 
     Vector<Core::ThreadStatistics> threads;