drm/xe: Drop xe_vm_assert_held() macro definition from xe_bo.h

It is already defined in xe_vm.h and shouldn't be duplicated.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240405113844.803-1-michal.wajdeczko@intel.com