staging: lustre: remove ENTRY/EXIT_NESTING stuff

These macros were empty, so remove them in anticipation of removing the
entire ENTRY/EXIT macros.

Cc: Peng Tao <tao.peng@emc.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>