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.

Kernel/API/Syscall.h

@@ -157,7 +157,6 @@ namespace Kernel {
     S(dbgputstr)              \
     S(watch_file)             \
     S(shbuf_allow_all)        \
-    S(set_process_icon)       \
     S(mprotect)               \
     S(realpath)               \
     S(get_process_name)       \