d/rules.real: Unset KBUILD_HOSTCFLAGS etc. instead of overriding to be empty

The upstream Makefile tries to add some flags to KBUILD_HOSTCFLAGS,
and now fails to build anything if we override it to be empty:

    .../scripts/kconfig/util.c:11:10: fatal error: hashtable.h: No such file or directory
       11 | #include <hashtable.h>
          |          ^~~~~~~~~~~~~
    compilation terminated.

Instead of overriding these flags variables to be empty, unset them in
the environment.

debian/changelog

@@ -28,6 +28,8 @@ linux (6.11~rc4-1~exp1) UNRELEASED; urgency=medium
       - Adjust context in "security,perf: Allow further restriction of
         perf_event_open"
   * [rt] Update to 6.11-rc3-rt3
+  * d/rules.real: Unset KBUILD_HOSTCFLAGS etc. instead of overriding to be
+    empty
  -- Salvatore Bonaccorso <carnil@debian.org>  Mon, 12 Aug 2024 20:56:44 +0200

debian/rules.real

@@ -32,11 +32,12 @@ undefine LANGUAGE
 stamp = [ -d $(dir $@) ] || mkdir $(dir $@); touch $@
 
 setup_env := env -u ABINAME -u ARCH -u FEATURESET -u FLAVOUR -u VERSION -u LOCALVERSION
+# XXX: All the tools leak flags between host and build all the time, just don't care.  See #1050991.
+setup_env += -u KBUILD_HOSTCFLAGS -u HOSTCFLAGS -u KBUILD_HOSTLDFLAGS
 setup_env += DISTRIBUTION_OFFICIAL_BUILD=1 DISTRIBUTOR="$(DEB_VENDOR)" DISTRIBUTION_VERSION="$(SOURCEVERSION)" KBUILD_BUILD_TIMESTAMP="@$(SOURCE_DATE_EPOCH)" KBUILD_BUILD_VERSION_TIMESTAMP="$(DEB_VENDOR) $(SOURCEVERSION) ($(SOURCE_DATE_UTC_ISO))" KBUILD_BUILD_USER="$(word 1,$(subst @, ,$(MAINTAINER)))" KBUILD_BUILD_HOST="$(word 2,$(subst @, ,$(MAINTAINER)))"
 setup_env += KBUILD_VERBOSE=$(if $(filter terse,$(DEB_BUILD_OPTIONS)),0,1)
 
-# XXX: All the tools leak flags between host and build all the time, just don't care.  See #1050991.
-MAKE_CLEAN = $(setup_env) $(MAKE) KCFLAGS=-fdebug-prefix-map=$(CURDIR)/= KBUILD_HOSTCFLAGS='' HOSTCFLAGS='' KBUILD_HOSTLDFLAGS=''
+MAKE_CLEAN = $(setup_env) $(MAKE) KCFLAGS=-fdebug-prefix-map=$(CURDIR)/=
 MAKE_SELF := $(MAKE) -f debian/rules.real $(MAKEOVERRIDES)
 MAKEOVERRIDES =