arch: arc: threads: Comment clean-up

commit 780324b ("cleanup: rename fiber/task -> thread")
seems to be done by a script and in that particular case turned
menaingful sentence into nonsense. Alas, threads might be in all
four states.

Signed-off-by: Alexey Brodkin <[email protected]>
Cc: Anas Nashif <[email protected]>

Author: abrodkin

arch/arc/include/kernel_arch_thread.h

@@ -20,8 +20,7 @@
 #define ZEPHYR_ARCH_ARC_INCLUDE_KERNEL_ARCH_THREAD_H_
 
 /*
- * Reason a thread has relinquished control: threads can only be in the NONE
- * or COOP state, threads can be one in the four.
+ * Reason a thread has relinquished control.
  */
 #define _CAUSE_NONE 0
 #define _CAUSE_COOP 1