[TASK] Extbase (Persistence): Remove duplicate registration in identity map

Remove identity-map registration from
Tx_Extbase_Persistence_Repository->findByUid()
since the object has already been registered in the DataMapper
in mapSingleRow().

Thanks to Alexander Stehlik.

Change-Id: I76382f6f12cbcc290f70cd52a465f2c92f7bf4c2
Resolves: #26875