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/LibC/unistd.h

@@ -61,7 +61,6 @@ int fsync(int fd);
 void sysbeep();
 int gettid();
 int donate(int tid);
-int set_process_icon(int icon_id);
 int getpagesize();
 pid_t fork();
 int execv(const char* path, char* const argv[]);