examples: remove call to exit from thread function

The exit call in the thread functions have potential thread racing
issues and may leave another process hanging. I think it is unnecessary
for the example and may cause more confusion.

examples/developers/threads.c

@@ -91,7 +91,6 @@ void *thrdsub(void *arg)
             MPI_Recv(r_buff, 1024, MPI_CHAR, 1, mythrdrank, MPI_COMM_WORLD, &status);
             if (strcmp(s_buff, r_buff) != 0) {
                 printf("comm_rank=%d mythrdrank=%d bad recv\n", comm_rank, mythrdrank);
-                exit(-1);
             }
             if (thread_level_provided == MPI_THREAD_SINGLE ||
                 thread_level_provided == MPI_THREAD_FUNNELED) {
@@ -106,7 +105,6 @@ void *thrdsub(void *arg)
             MPI_Recv(r_buff, 1024, MPI_CHAR, 0, mythrdrank, MPI_COMM_WORLD, &status);
             if (strcmp(s_buff, r_buff) != 0) {
                 printf("comm_rank=%d mythrdrank=%d bad recv\n", comm_rank, mythrdrank);
-                exit(-1);
             }
             MPI_Send(s_buff, strlen(s_buff) + 1, MPI_CHAR, 0, mythrdrank, MPI_COMM_WORLD);
             if (thread_level_provided == MPI_THREAD_SINGLE ||