LU-2675 libcfs: remove {ENTRY,EXIT}_NESTING macros

LU-2675 libcfs: remove {ENTRY,EXIT}_NESTING macros

Remove the unnecessary macros ENTRY_NESTING_SUPPORT, ENTRY_NESTING,
EXIT_NESTING, and __current_nesting_level().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I12c3c9a253b1dc0929a78c2f807b2a94434d3496
Reviewed-on: http://review.whamcloud.com/11984
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>