database: stop using incremental selectors
database: stop using incremental selectors

There is a bug in incremental_selector for partitioned_sstable_set, so
until it is found, stop using it.

This degrades scan performance of Leveled Compaction Strategy tables.

Fixes #3513. (as a workaround)
Introduced: 2.1
Message-Id: <6dbb51b342934b52df1e8e173bb645094812e848@scylladb.com>

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