Replace calls to slurm_conf_lock() with direct reference to slurmctld_conf in slurmctld.

slurm_conf_lock() returns &slurmctld_conf (with added locking).
Avoid the additional function call and refernce the variable directly.

Locking the conf is already handled within slurmctld by lock_slurmctld,
and locations within slurmcltd changing slurmctld_conf don't grab the separate
slurm_conf_lock() mutex anyways.