CLEANUP: acme: stored value is overwritten before it can be used

CLEANUP: acme: stored value is overwritten before it can be used

   >>>     CID 1609049:  Code maintainability issues  (UNUSED_VALUE)
   >>>     Assigning value "NULL" to "new_ckchs" here, but that stored value is overwritten before it can be used.
   592             struct ckch_store *old_ckchs, *new_ckchs = NULL;

Coverity reported an issue where a variable is initialized to NULL then
directry overwritten with another value. This doesn't arm but this patch
removes the useless initialization.

Must fix issue #2932.