We don't need to add module/\*headerpath\*.

We can just dump janet.h into the lib folder as well.

src/core/core.janet

@@ -1605,10 +1605,6 @@ value, one key will be ignored."
   on Windows is C:/Janet/Library."
   (or (os/getenv "JANET_PATH") ""))
 
-(var module/*headerpath*
-  "The path where janet/janet.h is located."
-  (or (os/getenv "JANET_INCLUDEDIR") ""))
-
 (defn- fexists [path]
   (def f (file/open path))
   (if f (do (file/close f) path)))