refactor: remove dead M4MODPATH code.

* m4/m4private.h (USER_MODULE_PATH_ENV): Remove.
* m4/module.c: Correct doc-comment header details.
(m4__module_init): Remove PKGLIBEXECDIR and M4MODPATH handling.
Move configmake.h include from here...
* m4/path.c: ...to here.
(m4__include_init): Append PKGLIBDIR to search path.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>