Remove duplicated tests

Those tests were removed because...

serde-migrated:
- test_doctype:             not testing anything useful - doctype not parsed because
                            it is deprecated technology
- test_parse_i64:           covered by `serde-de::trivial` tests
- test_parse_u64:           covered by `serde-de::trivial` tests
- test_parse_bool:          covered by `serde-de::trivial` tests
- test_parse_unit:          covered by `serde-de::unit` tests
- test_parse_f64:           covered by `serde-de::trivial` tests
- test_parse_struct:        covered by `serde-de::struct_` tests
- test_amoskvin:            covered by `serde-de::struct_` and `serde-de::seq` tests
- test_nicolai86:           covered by `serde-de::struct_` and `serde-de::seq` tests.
                            Sequence fields should me marked as `#[serde(default)]`
                            or be wrapped in Option to be able to deserialize from
                            empty list
- test_hugo_duncan2:        covered by `serde-de::struct_` and `serde-de::seq` tests
- test_hugo_duncan:         covered by `serde-de::struct_` and `serde-de::unit` tests
- test_parse_xml_value:     covered by `serde-de::explicit_value` test
- test_parse_complexstruct: test 1 just incorrect; tests 2 and 3 covered by
                            `serde-de::unit`, `serde-de::tuple`, and
                            `serde-de::seq` tests
- test_parse_attributes:    covered by `serde-de::explicit_value`, `serde-de::struct_`
                            and `serde-de::nested_struct` tests
- test_parse_hierarchies:   covered by `serde-de::struct_`, `serde-de::nested_struct`
                            and `serde-de::seq` tests. Sequence fields should me marked
                            as `#[serde(default)]` or be wrapped in Option to be able to
                            deserialize from empty list
- unknown_field:            covered by `serde-de::struct_` and `serde-de::seq` tests
- futile:                   covered by `serde-de::trivial` tests
- futile2:                  covered by `serde-de::trivial` tests

serde_roundtrip:
- basic_struct:             covered by `serde-de::struct_` tests
- no_contiguous_fields:     covered by `serde-de::seq` tests
- escapes_in_cdata:         covered by `serde-de::trivial::struct::cdata` tests