[mle] remove unused attach state kAttachStateSynchronize (#3542)

The attach state `kAttachStateSynchronize` was intended for sending
"Child Update Requests" after a reset (if device was previously
attached) using the `mAttachTimer` for retransmissions. However,
"Child Update" transmissions use `mMessageTransmissionTimer`
instead.

src/core/thread/mle.cpp

@@ -272,7 +272,6 @@ otError Mle::Start(bool aAnnounceAttach)
     }
     else
     {
-        SetAttachState(kAttachStateSynchronize);
         mChildUpdateAttempts = 0;
         SendChildUpdateRequest();
     }
@@ -1635,10 +1634,6 @@ void Mle::HandleAttachTimer(void)
         assert(false);
         break;
 
-    case kAttachStateSynchronize:
-        SendChildUpdateRequest();
-        break;
-
     case kAttachStateProcessAnnounce:
         ProcessAnnounce();
         break;
@@ -4153,10 +4148,6 @@ const char *Mle::AttachStateToString(AttachState aState)
         str = "Idle";
         break;
 
-    case kAttachStateSynchronize:
-        str = "Sync";
-        break;
-
     case kAttachStateProcessAnnounce:
         str = "ProcessAnnounce";
         break;

src/core/thread/mle.hpp

@@ -1082,7 +1082,6 @@ protected:
     enum AttachState
     {
         kAttachStateIdle,                ///< Not currently searching for a parent.
-        kAttachStateSynchronize,         ///< Looking to synchronize with a parent (after reset).
         kAttachStateProcessAnnounce,     ///< Waiting to process a received Announce (to switch channel/pan-id).
         kAttachStateStart,               ///< Starting to look for a parent.
         kAttachStateParentRequestRouter, ///< Searching for a Router to attach to.