Remove invalid validation of default included_resources

[sliverc]

Fixes #949

Description of the Change

#900 inadvertently introduced validation of default included_resources which does not work properly with nested included resources (such as when a default included resources also defines its own included resources).

This PR reverts this validation but leaving the default prefetching of default included resources as intended in PR #900.

This is a quick fix to get this regression solved. In the long run we need to think whether it makes sense at all to do validation of include in init of the serializer anyway. But can be discussed later.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #956 (49be826) into master (74d2ff8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #956   +/-   ##
=======================================
  Coverage   96.81%   96.81%           
=======================================
  Files          64       64           
  Lines        3888     3888           
=======================================
  Hits         3764     3764           
  Misses        124      124           

Impacted Files	Coverage Δ
example/serializers.py	88.07% <ø> (ø)
example/tests/test_serializers.py	100.00% <ø> (ø)

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[n2ygk]

👍