doc/libav-merge: remove the hls merge TODO

This TODO is done.
See 5caaa3a4Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Steven Liu <lq@onvideo.cn>

doc/libav-merge.txt

@@ -105,7 +105,6 @@ Collateral damage that needs work locally:
 - Merge proresdec2.c and proresdec_lgpl.c
 - Merge proresenc_anatoliy.c and proresenc_kostya.c
 - Fix MIPS AC3 downmix
-- hlsenc encryption support may need some adjustment (see edc43c571d)
 
 Extra changes needed to be aligned with Libav:
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