Bug 5194: Remove all unused debug sections (#969)

Bug 5194: Remove all unused debug sections (#969)

... that we can find quickly.

Naming/documenting debugging sections is a good idea, but that should
not be done in every source code file that relies on that section. We
cannot remove all such DEBUG: declarations without developing a proper
way to name/document sections, but we can (and, given Bug 5194
existence, probably should) remove the unused ones -- their removal does
not remove any immediately usable info.

Bug 5194 report was specific to Section 56: Folks misinterpret section
56 "HTTP Message Body" title as promising to dump message bodies to
cache.log, which is not a functionality that should be driven by a
debugging section. Currently, Squid lacks such functionality.