collator-protocol: remove `elastic-scaling-experimental` feature (#4595)

Validators already have been upgraded so they could already receive the
new `CollationWithParentHeadData` response when fetching collation.
However this is only sent by collators when the parachain has more than
1 core is assigned.

TODO:
- [x] PRDoc

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Signed-off-by: Andrei Sandu <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>