Unconditionally reset load-path after dumping

Unconditionally reset load-path after dumping

* src/lread.c: (dump_path): Remove.
(load-path-default): Remove `changed' argument.
Do not set dump_path permanently.  Simplify.
(init_lread): Simplify.
(syms_of_lread): Remove dump_path.

* lisp/loadup.el (load-path): Warn if site-load or site-init changes it.
No more need to reset it when bootstrapping.

* doc/lispref/internals.texi (Building Emacs):
* doc/lispref/loading.texi (Library Search): Mention that site-load,
site-init cannot change load-path.

* INSTALL: No longer mention load-path and site-init/site-load.

* etc/NEWS: Mention this.

Fixes: debbugs:16107