LU-6142 ptlrpc: remove bd_import_generation field.

LU-6142 ptlrpc: remove bd_import_generation field.

This field is set, but never accessed. So remove it.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I26791e9fbc50676a705a94755674945eea9bf262
Reviewed-on: https://review.whamcloud.com/35880
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Arshad Hussain <arshad.super@gmail.com>