reference: Remove mention of unit type

src/doc/reference.md

@@ -3437,14 +3437,6 @@ of the tuple. It has no nominal name and is instead structurally typed.
 Tuple types and values are denoted by listing the types or values of their
 elements, respectively, in a parenthesized, comma-separated list.
 
-The empty tuple is the "unit" type `()`, having the single value `()`
-(occasionally called "nil"). [^unittype]
-
-[^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
-    reference variables; the "unit" type is the implicit return type from functions
-    otherwise lacking a return type, and can be used in other contexts (such as
-    message-sending or type-parametric code) as a zero-size type.]
-
 Because tuple elements don't have a name, they can only be accessed by
 pattern-matching or by using `N` directly as a field to access the
 `N`th element.