vircgroup: Remove unused variables in virCgroupV2Available

vircgroup: Remove unused variables in virCgroupV2Available

After recent commit 'contFile' and 'contStr' became unused breaking
build with clang:

../../../libvirt/src/util/vircgroupv2.c:72:26: error: unused variable 'contFile' [-Werror,-Wunused-variable]
        g_autofree char *contFile = NULL;
                         ^
../../../libvirt/src/util/vircgroupv2.c:73:26: error: unused variable 'contStr' [-Werror,-Wunused-variable]
        g_autofree char *contStr = NULL;
                         ^

Fixes: a0f37232b9c4296ca16955cc625f75eb848ace39
Signed-off-by: Peter Krempa <pkrempa@redhat.com>