MEDIUM: haproxy: don't use sync_poll_loop() anymore in the main loop

MEDIUM: haproxy: don't use sync_poll_loop() anymore in the main loop

This partially reverts commit d8fd2af ("BUG/MEDIUM: threads: Use the sync
point to check active jobs and exit") which used to address an issue in
the way the sync point used to check for present threads, which was later
addressed by commit ddb6c16 ("BUG/MEDIUM: threads: Fix the exit condition
of the thread barrier"). Thus there is no need anymore to use the sync
point for exiting and we can completely remove this call in the main loop.