bugfix: plug a memleak in imuxsock

The hash table for the system socket was allocated twice. The other
one being in activateListeners().

This commit practically reverts: 34a77cde2423303da72ab773128a2ddcf41
The issue seems to be that the hash table is not initialized (to NULL)
rather then not being allocated.
bugfix: plug a memleak in imuxsock

Hash table and rate limiter for the system socket weren't freed.

A side-effect was that the "Last message repeated..." notice didn't
get printed if rsyslog was shut down before the repetition was broken.
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