dnsdist: Fix minor issues reported by Coverity

[rgacogne]

Short description

Fix some minor issues reported by Coverity:

• one useless test due to the condition being always true ;
• uncaught exception if we ever try to get the handle of a non-established TCP connection which should never happen.

Also remove some code that has been unused since we moved to an event-based logic.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)