Deprecate format_raw_length #6018
Description
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.