std.time.epoch: delete YearLeapKind

[wooster0]

There doesn't really seem to be a point in having this type.
Code seems to be improved without it.

Skimming through code that references the type I could find like 2-3 actual uses of it so it probably doesn't need a deprecation period.

[rohlem]

Not a big deal, but I personally don't really see this as an improvement. getDaysInMonth(false, .feb) is less descriptive.

For bool -> kind we could add a factory function callable as decl literal .fromBool(is) / .fromIsLeapYear(is), for the other way a method .isLeapYear().
(In my own code base I'd use this to get rid of all other @enumFromInt / @intFromEnum outside of these method implementations.)

Getting rid of unnecessary @as is unrelated, but nice.

[alexrp]

#21280