MINOR: task: remove __tasklet_remove_from_tasklet_list()

MINOR: task: remove __tasklet_remove_from_tasklet_list()

This function is only used at a single place directly within the
scheduler in run_tasks_from_lists() and it really ought not be called
by anything else, regardless of what its comment says. Let's delete
it, move the two lines directly into the call place, and take this
opportunity to factor the atomic decrement on tasks_run_queue. A comment
was added on the remaining one tasklet_remove_from_tasklet_list() to
mention the risks in using it.

(cherry picked from commit 2da4c316c2cb7b01f54ed1959e91d1799d13959c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 9a0374aaaafefa07b80e761f97f0fcd7688dabf4)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit e4480476c172b13e0bb072ef1afd31b359cc3917)
[wt: needed for next fix; context hanges in 2.1, no
     run_tasks_from_lists(), single LIST_DEL location]
Signed-off-by: Willy Tarreau <w@1wt.eu>