s390: remove odd comment

In the meantime hopefully most people got used to forward
declarations, therefore remove the explanation.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

Author: hcahca

arch/s390/include/asm/processor.h

@@ -228,7 +228,6 @@ typedef struct thread_struct thread_struct;
 	execve_tail();							\
 } while (0)
 
-/* Forward declaration, a strange C thing */
 struct task_struct;
 struct mm_struct;
 struct seq_file;