Remove num_days_from_epoch

Deciding between zero-indexed (slightly more reasonable, more compatible with
the outside world) and one-indexed (same indexing as `num_days_from_ce`) seems
not worth it. It's trivial to build `num_days_from_epoch` based on the docs in
`num_days_from_ce`, which punts on the decision and is therefor probably the
right decision.

src/lib.rs

@@ -915,21 +915,6 @@ pub trait Datelike: Sized {
         ndays += ((year * 1461) >> 2) - div_100 + (div_100 >> 2);
         ndays + self.ordinal() as i32
     }
-
-    /// Return the number of days since the unix epoch, 1970-01-01
-    ///
-    /// # Example:
-    ///
-    /// ```
-    /// use chrono::{NaiveDate, Datelike};
-    ///
-    /// assert_eq!(NaiveDate::from_ymd(1970, 1, 1).num_days_from_epoch(), 0);
-    /// assert_eq!(NaiveDate::from_ymd(1969, 1, 1).num_days_from_epoch(), -365);
-    /// assert_eq!(NaiveDate::from_ymd(1999, 12, 31).num_days_from_epoch(), 10_956);
-    /// ```
-    fn num_days_from_epoch(&self) -> i32 {
-        self.num_days_from_ce() - EPOCH_NUM_DAYS_FROM_CE
-    }
 }
 
 /// The common set of methods for time component.