CLEANUP: haproxy: rm no longer used mworker_reexec_waitmode

CLEANUP: haproxy: rm no longer used mworker_reexec_waitmode

This a first commit to prepare the removal of MODE_MWORKER_WAIT support. It has
became redundant with MODE_MWORKER, due to moving master-worker fork in init().
Master process does no longer perform reexec to free additional memory after
forking and does no longer changing its mode to MODE_MWORKER_WAIT, where it has
entered to its wait polling loop and has handled signals. Now, master enters in
this loop almost immediately after forking a worker and being always in mode
MODE_MWORKER.

So, we can remove mworker_reexec_waitmode() wrapper, which was used to set
HAPROXY_MWORKER_WAIT_ONLY variable and to call mworker_reexec(). But let's keep
for the moment the logic of reexec_on_failure() atexit callback for master in
order if in the future we will need to support this case again.