LU-10467 libcfs: remove l_wait_event() and related macros.

LU-10467 libcfs: remove l_wait_event() and related macros.

Now that all usage of l_wait_event() has been converted to standard
wait_event* macros, we can remove l_wait_event and related macros.

Also clean up a few comments that mentioned these removed macros.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ie5fdc21ac94520e52021aa870d351230d39b0bc7
Reviewed-on: https://review.whamcloud.com/37729
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>