fix(core): drop the unused Config forward declaration from HashRouter.h

HashRouter.h declared `class Config;` and never referred to it: the only
other mentions of the name in the file are two prose comments. Nothing
that includes this header depends on it either -- of the eighteen files
that do, the only one that names the type needs the full definition and
includes it itself. The declaration is vestigial, left behind when
setupHashRouter() moved to setup_HashRouter.h, which carries its own.

Left alone it is now an error rather than dead code. This branch's
SlotCensus work made Overlay.h include PeerfinderManager.h, because
Overlay::slotCensus() returns a SlotCensus by value. ValidatorList.cpp
includes both Overlay.h and HashRouter.h, and is the one translation unit
that reaches them without also pulling in xrpld/core/Config.h -- so
PeerFinder::Config and this declaration meet there, and
bugprone-forward-declaration-namespace reports a declaration with no
definition beside a same-named class in another namespace.

Removing it rather than including the real header: xrpl::Config lives in
xrpld, so an include would point the xrpl.libxrpl.core layer at the
layer above it.

include/xrpl/core/HashRouter.h

@@ -73,8 +73,6 @@ any(HashRouterFlags flags)
     return static_cast<std::underlying_type_t<HashRouterFlags>>(flags) != 0;
 }
 
-class Config;
-
 /**
  * Routing table for objects identified by hash.
  *