logilab-database #739824 [sqlserver] dropping unique index should not fail if it is unknown to the backend [validation pending]

prioritynormal
typebug
done in1.13.4
load1.000
load left0.000
closed by<not specified>
patch[sqlserver] Fix `index_exists` predicate for unique indexes [applied][sqlserver] drop unique index if it exists [folded][sqlserver] drop unique index if it exists [folded][sqlserver] Fix `index_exists` predicate for unique indexes [folded]