LU-9519 utils: liblustreapi header cleanup

LU-9519 utils: liblustreapi header cleanup

Remove superfluous 'external' qualifier from liblustreapi method prototypes.
Remove superfluous 'const' qualifier.

Test-Parameters: trivial
Change-Id: I818d5d2c9ae69d947f72c9306125715547714770
Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-on: https://review.whamcloud.com/27155
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>