remove unhelpful/outdated/unused doc file

doc/codegen.md

@@ -1,33 +0,0 @@
-# Code Generation
-
-## Data Representation
-
-Every type has a "handle". If a type is a simple primitive type such as i32 or
-f64, the handle is "by value", meaning that we pass around the value itself when
-we refer to a value of that type.
-
-If a type is a container, error union, optional type, slice, or array, then its
-handle is a pointer, and everywhere we refer to a value of this type we refer to
-a pointer.
-
-Parameters and return values are always passed as handles.
-
-Error union types are represented as:
-
-    struct {
-        error: u32,
-        payload: T,
-    }
-
-Optional types are represented as:
-
-    struct {
-        payload: T,
-        is_non_null: u1,
-    }
-
-## Data Optimizations
-
-Optional pointer types are special: the 0x0 pointer value is used to represent a
-null pointer. Thus, instead of the struct above, optional pointer types are
-represented as a `usize` in codegen and the handle is by value.