Some test files for flat container adaptors redundantly include
`<iostream>` and, surprisingly, `<cstddef>`. This patch removes the
redundant inclusions.
Inclusions of `<vector>` is also removed since a sane implementation is
expected to make instantiation of `flat_(multi)map<K, V>` or
`flat_(multi)set<K>` valid when only `<flat_map>` or `<flat_set>` is
included.