remove deprecated Did You Mean library settings

The Did You Mean enhancements in bug 1997485 deprecated the following six library settings and moved them to become attributes of the search class:

opac.did_you_mean.low_result_threshold
opac.did_you_mean.max_suggestions
search.symspell.keyboard_distance.weight
search.symspell.min_suggestion_use_threshold
search.symspell.pg_trgm.weight
search.symspell.soundex.weight

Those library settings were removed from the seed data and will not show up in new installations, but they were not removed outright from existing Evergreen databases on the theory that the Evergreen admin might want to review them before committing to the values to use for the search classes.

However, since those settings now do nothing, they should be removed, either for a 3.11.x point release or, at the latest, 3.12.