BUG/MEDIUM: task/h2: add an idempotent task removal fucntion

BUG/MEDIUM: task/h2: add an idempotent task removal fucntion

Previous commit 3ea351368 ("BUG/MEDIUM: h2: Remove the tasklet from the
task list if unsubscribing.") uncovered an issue which needs to be
addressed in the scheduler's API. The function task_remove_from_task_list()
was initially designed to remove a task from the running tasklet list from
within the scheduler, and had to be used in h2 to abort pending I/O events.
However this function was not designed to be idempotent, occasionally
causing a double removal from the tasklet list, with the second doing
nothing but affecting the apparent tasks count and making haproxy use
100% CPU on some tests consisting in stopping the client during some
transfers. The h2_unsubscribe() function can sometimes be called upon
stream exit after an error where the tasklet was possibly already
removed, so it.

This patch does 2 things :
  - it renames task_remove_from_task_list() to
    __task_remove_from_tasklet_list() to discourage users from calling
    it. Also note the fix in the naming since it's a tasklet list and
    not a task list. This function is still uesd from the scheduler.
  - it adds a new, idempotent, task_remove_from_tasklet_list() function
    which does nothing if the task is already not in the tasklet list.

This patch will need to be backported where the commit above is backported.