Deprecate `format_raw_length`

The `format_raw_length` option was introduced in 2015 (beets 1.3.16) to maintain backwards compatibility when the at this point new `DurationType` began displaying track lengths as MM:SS instead of raw seconds. This gave users a way to preserve the old behavior.

Today, however, the option is effectively undocumented, mentioned only once in the changelog, and has little practical use. The `beet list length:*` command already accepts both raw seconds and duration strings, and users who need unformatted values can access the raw length field directly. This makes the config option redundant.

A reasonable path forward would be to deprecate `format_raw_length`, warning users if it appears in configuration files, and then remove it entirely in version `3.0.0`.