Remove undocumented constants from Time::Seconds

Remove Time::Seconds::ONE_REAL_MONTH and Time::Seconds::ONE_REAL_YEAR which seems to me both undocumented and unusable. [These](https://github.com/rjbs/Time-Piece/blame/master/Seconds.pm#L30) [constants](https://github.com/rjbs/Time-Piece/blame/master/Seconds.pm#L32) were added in Time::Object 1.00 and since then nobody touched them (nor documented them).

If I'm wrong just ingore this PR and please add an example to the documentation.