decoder: flush directly from Delete(), don't flush if drained

Flush directly from Delete(), and don't flush if successfully drained (will be useful for gapless).

Partial revert of !3382 (merged) Cc. @alexandre-janniaux

The big question is: should vlc_input_decoder_Delete() require a prior call to vlc_input_decoder_Flush() or not ?

Fixes #28463 (closed)