LU-10291 lnd: remove concurrent_sends tunable

LU-10291 lnd: remove concurrent_sends tunable

Concurrent sends tunable was intended to limit the number of in-flight
transfers per connection. However queue depth does the exact same job.
So for example if the queue depth is negotiated to 16 and
concurrent_sends is set to 32, the maximum number of in-flight
transfers doesn't exceed 16. There is no need to keep concurrent_sends
around since it doesn't add any unique functionality

Test-Parameters: trivial
Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: I4dd7f5ecac5a7d10403c4f0ab0f01374a2f10206
Reviewed-on: https://review.whamcloud.com/30312
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Doug Oucharek <dougso@me.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>