The features removed are:
* mkvmerge: the options `--identify-verbose` (and its counterpart
`-I`), `--identify-for-gui`, `--identify-for-mmg` and
`--identification-format verbose-text`
* all command line tools: the old, proprietary format used for option
files
* all command line tools: support for passing command line options via the
environment variables `MKVTOOLNIX_OPTIONS`, `MKVEXTRACT_OPTIONS`,
`MKVINFO_OPTIONS`, `MKVMERGE_OPTIONS`, and `MKVPROPEDIT_OPTIONS`