Change IPv6 output format to no longer automatically be bracketed.

Instead, the `addr_to_uri` script-level function can be used to
explicitly add brackets to an address if it's IPv6 and will be
included in a URI or when a ":<port>" needs to be appended to it.

src/logging/WriterBackend.cc

@@ -261,7 +261,7 @@ string WriterBackend::Render(const threading::Value::addr_t& addr) const
 		if ( ! bro_inet_ntop(AF_INET6, &addr.in.in6, s, INET6_ADDRSTRLEN) )
 			return "<bad IPv6 address conversion>";
 		else
-			return string("[") + s + "]";
+			return s;
 		}
 	}