Remove code for starting a otp scan (multiple commits)

[mattmundell]

Same as Jan's #608 but using multiple commits so that reviewers can read it.

I compared this manage.c and the manage.c from 608 and they are the same.

Here's the description from 608:

In run_task(), do not call anymore run_otp_task() as fall-though
for non-matching scanner type (not the case anyway).
Instead return a error code in case of an unknown scanner type.

Subsequently remove the now-unsed function run_otp_task().
And further delete all functions that become usused by this.

The diff looks complicated, but actually it is only changing
run_task() and other than that simply remove entire functions.

[timopollmeier]

Thanks, this is much easier to read. I already did some test runs with #608 and after having a clearer look at the changes it still looks good to me.