Remove deprecated irept::get_unsigned_int

[tautschnig]

It has been deprecated since 06/2018 and has no users. This enables removing the
deprecate.h include. Also clean up an outdated NOLINT.

(Of course inspired by #4236.)

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[kroening]

Do we want to form a view how long something needs to be deprecated to get deleted?
Do we have something like 'lives until release X'?
Could be made dependent on effort to refactor.

[tautschnig]

Do we want to form a view how long something needs to be deprecated to get deleted?

Some time ago you suggested it should be at least 6 months, so that's what I'm currently using as my guideline. But also I agree that the cost for downstream users should be taken into account, i.e., if it requires a lot of change then we ought to give people more time.