Fix serialization cache bypass in _serialize_included

[csabella]

Summary

• The merge conflict resolution in d68f0ce kept both sides of a conflict in _serialize_included(), causing every included resource to be serialized fresh even when a cached result exists
• The cached result was computed but immediately overwritten by an unconditional schema.dump() call on the next line
• Removes the duplicate lines so the cache is actually used
• Bumps version to 0.24.1

Before (broken)

data = result                          # result from cache
item = data["data"]                    # extracts item from cache
result = self.schema.dump(value)       # DUMPS AGAIN unconditionally
item = result["data"]                  # OVERWRITES cached item

After (fixed)

item = result["data"]

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