MINOR: task: split the tasklet vs task code in process_runnable_tasks()

MINOR: task: split the tasklet vs task code in process_runnable_tasks()

There are a number of tests there which are enforced on tasklets while
they will never apply (various handlers, destroyed task or not, arguments,
results, ...). Instead let's have a single TASK_IS_TASKLET() test and call
the tasklet processing function directly, skipping all the rest.

It now appears visible that the only unneeded code is the update to
curr_task that is never used for tasklets, except for opportunistic
reporting in the debug handler, which can only catch si_cs_io_cb,
which in practice doesn't appear in any report so the extra cost
incurred there is pointless.

This change alone removes 700 bytes of code, mostly in
process_runnable_tasks() and increases the performance by about
1%.