demux_lavf: remove `fix_editlists` hack for mp4

Remove this "hack" from 2017 which disabled `advanced_editlist` while
demuxing mov/mp4. See: b7b12c36af96af14b8371d16c5a7f98a7a17915f as well
as b86a2316df260de70012fb05dbddf06118b00611

At present, advanced edit list support seems to be mature enough that it
doesn't need to be disabled by us and disabling it seems to break more
files based on testing with `elist` files in FFmpeg's FATE suite.