[dnssd-server] remove unused using for HostSwap16() (#6758)

This addresses `clang-tidy` warning for unused `using` statement. This
commit also ensures `OT_DNSSD_SERVER` is enabled in `make-pretty`
script and therefore covered by github action `build pretty`
workflow.

script/make-pretty

@@ -95,6 +95,7 @@ readonly OT_CLANG_TIDY_BUILD_OPTS=(
     '-DOT_DHCP6_SERVER=ON'
     '-DOT_DIAGNOSTIC=ON'
     '-DOT_DNS_CLIENT=ON'
+    '-DOT_DNSSD_SERVER=ON'
     '-DOT_DUA=ON'
     '-DOT_MLR=ON'
     '-DOT_ECDSA=ON'

src/core/net/dnssd_server.cpp

@@ -43,8 +43,6 @@
 #include "net/srp_server.hpp"
 #include "net/udp6.hpp"
 
-using ot::Encoding::BigEndian::HostSwap16;
-
 namespace ot {
 namespace Dns {
 namespace ServiceDiscovery {