Remove code for starting a otp scan.

[janowagner]

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.

[mattmundell]

Really hard to see what is changing here.

Also, I would expect a much more drastic impact from removing the running
of OTP tasks. A lot of the complications in the gvmd.c select loop are to
support listening to the OTP scanner while also handling GMP input, if I
remember right.

[janowagner]

Also, I would expect a much more drastic impact from removing the running
of OTP tasks. A lot of the complications in the gvmd.c select loop are to
support listening to the OTP scanner while also handling GMP input, if I
remember right.

Thats true and why I called it "remove code for starting...".
Since this diff was already a bit hard to read, I wanted to have separate PRs
for removing the code that handles a running OTP scan.

[mattmundell]

I added #623 because I couldn't follow this patch. Please can we merge that one instead?

[timopollmeier]

I've reviewed and merged #623, so I'm closing this one.