mkosi: Drop usage of _systemd_QUIET in arch build script

We dropped the variable in the packaging specs for Arch to keep the
integration points as minimal as possible so let's stop using it in
the build script as well.

mkosi.images/build/mkosi.conf.d/arch/mkosi.build.chroot

@@ -85,7 +85,6 @@ env --chdir="pkg/$PKG_SUBDIR" \
     $( ((WITH_TESTS)) || echo --nocheck) \
     --force \
     _systemd_UPSTREAM=1 \
-    _systemd_QUIET=$( ((MESON_VERBOSE)); echo $? ) \
     BUILDDIR="$PWD/pkg/$PKG_SUBDIR" \
     PKGDEST="$BUILDDIR" \
     PKGEXT=".pkg.tar" \