Deletes incorrect sentence

In numeric contexts, it will still return a Duration. If you do any
operation with other kind of numbers, the result will be that number's
type, but that's the operation, not just the context. Closes #1589

doc/Type/Duration.pod6

@@ -7,9 +7,7 @@
     class Duration is Cool does Real { }
 
 A C<Duration> represents a length of time in atomic seconds, with
-fractions. Like an L<Instant>, it is epoch-agnostic. In numeric contexts
-(situations where a C<Duration> is used as a number), a C<Duration> will return
-either a L<Rat> or a L<FatRat> representing the number of seconds it holds.
+fractions. Like an L<Instant>, it is epoch-agnostic.
 
 C<Duration>s can be subtracted from or added to C<Instant>s to yield another,
 new C<Instant>. Subtracting one C<Instant> from another yields a C<Duration>.